Beis Din needs to hear both sides

Shemos (23:1) You must not carry false rumors; you shall not join hands with the guilty to act as a malicious witness: 

Mishneh Torah, (Negative Mitzvot 281) Not to hear one of the parties to a suit in the absence of the other party, as it is said, “Thou shalt not receive a vain report” (Ex. 23:1).

Sefer HaChinukh (74:1) To not hear the claim of a litigant when it is not in front of his fellow litigant: That the judge not hear the claim of one, not in front of his adversary, as it is stated (Exodus 23:1), "You shall not raise a false report." And the reason is because people will speak idle words when not in front of their adversary. And the judge is commanded about this so that he not bring the untruths of one of them into his soul. And so does Mekhilta d'Rabbi Yishmael 23:1 come [to tell us] that this warning of "You shall not raise, etc." is said about this. And they also said there that it is also a warning to the litigant, for him too, not to make his claims to the judge not in front of his adversary, and even if the judge wants to hear it. And about this, it is also said (Exodus 23:7), "From a false matter, distance yourself." And they, may their memory be blessed, also said (Makkot 23a) that this negative commandment includes telling evil speech, and accepting it, and giving false testimony.

Tur, (Choshen Mishpat 17) אסור לדיין לשמוע בעל דין האחד קודם שיבא חבירו או שלא בפני חבירו:

https://shulchanaruchharav.com/halacha/knowing-how-to-judge-hearing-the-details-of-a-dispute-in-the-presence-of-both-sides/

It is forbidden[2] for a judge to hear the claims of one side of the case [plaintiff or defendant] not in the presence of the other side.[3] This prohibition applies equally for the plaintiff/defendant, that they may not make a claim in front of the judge unless the other person is present.[4] [The Zohar states that one who hears one side without the other is considered as if he has accepted upon himself a foreign G-d.[5]]

Bedieved: In the event that a judge transgressed, and heard one side of the argument not in the presence of the other, some Poskim[6] rule he may still judge the case. Other Poskim[7], however, question this allowance.

Shulchan Aruch (C.M. 17:5) It is forbidden to the Judge to hear the words [pleas] of one litigant in the absence of his fellow-litigant.21Yad, Sanhedrin XXI, 7. Derived from Sheb. 31a: ‘Whence do we know that a Judge must not hear the claims of one litigant before his fellow-litigant arrives? — For it is said: ‘From a false matter keep far (Ex. XXIII, 7).’ San. 7b: ‘Hear (the causes) between your brethren and judge righteously (Deut. I, 16). Said. R. Ḥanina, This is a warning to the Court of Law not to hear the claims of one litigant before his fellow-litigant arrives (Because in the absence of the other party he will not refrain from stating false pleas).’ However, if the Judge had already heard the claims of one litigant in the absence of the other litigant, it is permissible for him to try the case. But, if after listening to one litigant, he gave the latter a written ruling in accordance with the majority of the Codifiers and Responsa, then he is forbidden to try the case because he is considered an interested party, since he will not contradict his own ruling — ShaK, Tummim. , Zohar wa-Yesheb 179b: ‘Who is regarded as a cunning knave (רשע ערום)? — One who pleaded his cause before the Judge before his fellow-litigant arrived (cf. Sot. 21b), as it written, He that pleadeth his cause first seemeth just (the litigant who pleads first states every point in his favour and when he is through it looks as if he is right); but his neighbour cometh and searcheth him out (Prov. XVIII, 17, i.e., the statements of the second party appear false and the Judge searches him out. Hence, both parties should be heard together). This is forbidden even if one litigant states both his cause and that of his adversary — P.Tesh. Zikron Ya‘akob writes that if one litigant presented his case before one whom he knows will act as Judge in his case, i.e., the litigant stated: ‘So-and-so has done me violence,’ — the law is that if the Judge does not investigate into the circumstances surrounding the case under which the violence was committed, he is permitted subsequently to act as Judge. MaHaRIL, however, implies that even pleading in such a form is forbidden (v. D.M.) — P.Tesh. Gloss: And this applies only when the Judge knows that he will act as Judge in the case; but if he had [already] heard the pleas of one party and subsequently the second party was satisfied to be tried before him, he is permitted to act as Judge in the case.22MaHaRIL Resp. 195 — G. This means that the Judge is also aware that the other litigant will have to appear before him, in which case the law is that in the first instance he should not hear the cause of the single litigant for perhaps the second litigant will not be satisfied to be tried before him since the Judge had already heard the pleas of his adversary. Furthermore, when the Judge knows that he will act as Judge, the pleas of the litigant who appeared alone will make an impression upon him, which is not the case, however, where the Judge does not know that he will act as Judge, — then he is permitted to listen to one litigant alone in which case he may try the case only if he tells the second litigant that he had already heard the pleas of the other party, and the second litigant agrees to appear before him, although he heard the pleas of the first litigant — M.E. For this may be equated to a case where one agrees to be tried by a relative of the other party or one who is otherwise unfit to judge — A.H. Moreover, since he was unaware that he would act as Judge in the case, the pleas of the first litigant do not impress themselves upon him — A.H. No Sage should write a decision for one of the litigants in [the following] manner, [viz.,] 'If such [are the circumstances of the case which this litigant set forth before me, then the law would be as follows],'23 Aboth I, 8: ‘Do not make yourself to be like legal advisers (i.e., be careful as Judge not to suggest an advice to one of the litigants).’ or [even] write for him [the litigant] his opinion without [giving] a [definitive] decision as long as he did not hear the words [claims] of both [litigants] lest from his [the Judge's] words they will learn to utter lies,24Aboth I, 9: ‘Simeon b. Shetaḥ would say: Be thorough in the examination of the witnesses and be cautious in your words lest from them (your words) they will learn to utter lies.’ Although this text deals with the interrogation of witnesses, v., however, Bertinora a.l. who refers this both to the witnesses and the litigants. Cf. also ibid. I, 8, Rashi (on the authority of R. Jehudai Gaon) and R. Jonah. [and] furthermore because later the second [litigant] will plead in a different manner and he [the Judge] will [then] be obliged to write the reverse and disrepute will be brought thereby on the Sage.25RaShBA and RIBaSh s. 179 — G. Although the ruling is that ‘we disregard any disrepute that may be brought on the Court’ (v. B.B. 31a; Ket. 26b), and consequently, in the present ruling we should not be concerned with the fact that the Judge on hearing the second litigant will have to alter his judgment and thereby bring contempt on himself, nevertheless, this applies only ex post facto where the Judge is compelled to give a verdict, but not in the present case where he is not obliged to hear the litigants separately. Hence, in the latter case we are afraid of bringing contempt on the Judge. , W.G. and Be’er Eliyahu. 

However, if the Judge notices that by not giving a decision a wrong might result, e.g., where the inquirer desires to dissolve an engagement or the like, or where dissension might ensue, or where he might save one from being defrauded — then he is obliged to give his decision — A.H. 

Likewise, is the litigant cautioned against [acting] thus.26San. 7b: ‘R. Kahana stated: This (v. first part of text supra n. 21) can be derived from, Thou shalt not take up (tissa) a false report (Ex. XXIII, 1, תשא) which may (also refer to the litigant) by reading tashshi (תשיא, the Hiph‘il from rt. נשא ‘to deceive’ or ‘to mislead,’ i.e., if one litigant pleads in the absence of his opponent the Judge might be mislead and won over).’ San. ibid.: ‘Hear (the causes) between your brethren and judge righteously (Deut. I, 16). Said R. Ḥanina, this is a warning … to the litigant not to explain his case to the Judge before his adversary appears. Shamo‘a (hear) can also be read shamme‘a (in the Pi‘el which has a causative meaning, i.e., ‘to make hear,’ ‘to explain’).’ In Sheb. 31a this is derived from From a false matter keep far (Ex. XXIII, 7). , also Sot. 21b. Cf. supra n. 21. If it happens that one of the litigants arrives first, he should not sit in the presence of the Judge so as not to be under suspicion — A.H. A scholar who has a case before his teacher should not arrive [at Court] first, [i.e.,] prior to his opponent so that it should not appear that he is [arriving first] in order to clarify his pleas in the absence of his fellow [-litigant]. However, if he has an appointed time to come and study before him and that time had arrived, he is permitted.27Sheb. 30b: ‘Rabbah b. Huna stated: If a scholar and an illiterate person are in dispute with one another, the scholar (and much more so a stranger who is not his student — M.E.) should not arrive first and sit down (before the Judge prior to the arrival of his opponent), because it will appear (i.e., he will be suspected even if he keeps his silence — Rashi) as if he is presenting his case. And we say this only where he has not an appointed time with him (to study); but if he has an appointed time with him, we do not care (i.e., if the Judge is his teacher and they have an appointed time to conduct their studies, then the scholar may come prior to the arrival of his opponent), because he (the opponent) will say, He is occupied with his appointment (for study).’

Aruch Hashulchan (CM 17:7) כתיב שמוע בין אחיכם ושפטתם וגו' מכאן שהדיין צריך לשמוע הטענות כששני הבע"ד עומדים לפניו ואסור לשמוע טענות מאחד כשאין השני אצלו ובזוהר וישב כתוב דכל דיינא דקביל מבר נש מלה עד לא ייתי חבריה כאלו מקבל עליו טעוא אחרא וכו' ועל זה נאמר לא תשא שמע שוא כי טענות של הראשון נקבע בלבו ויוכל להיות שמזה לא יצא הדין לאמתו ואם אירע שאחד סיפר טענותיו לפני חכם שלא ידע שיהיה דיין בדבר ואח"כ בקשוהו לדון בזה צריך החכם להגיד להשני שכבר שמע טענות הראשון ואם עכ"ז נתרצה השני רשאי להיות דיין בדבר דכיון שזה קיבל עליו לא גרע מקבלת קרוב או פסול ועוד דמאחר שלא ידע שיהיה דיין בדבר לא נקבעו הדברים בלבו שיטה אחריהם ומי שהוא דיין קבוע מהנכון ליזהר שלא לקבל דברים מאדם דרך סיפור כשמבין הדיין שיש לו ד"ת בזה ויכול לומר להמספר לו דע לך שבשמעי המעשה ממך א"א לי להיות דיין בדבר וכשם שאסור לדיין לשמוע מבע"ד אחד שלא בפני חבירו כמו כן חל האיסור על הבע"ד ולכן מהראוי ששני הבע"ד יבואו יחד לפני הדיינים ולא יבא האחד קודם כדי שלא יהא נראה כמקדים לסדר טענותיו ואם אירע שבא קודם לא ישב אצל הדיינים ואפילו תלמיד שיש לו דין לפני רבו לא יקדים לבא אא"כ היתה לו עת קבוע ללמוד עם רבו שרי דבזה ליכא חשדא:

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Gray Matter (II, Beit Din, Summons to Beit Din 56)  Due to the reported existence of unscrupulous batei din, we have chosen to add a few criteria by which to assess a beit din’s credibility. An honorable Beit Din must avoid conflicts of interest (Shulchan Aruch, C.M. 7:12 and 37:1), anything that even slightly resembles bribery (C.M. 9:1), and excessively high fees (C.M. 9:5).¹¹Also see Teshuvot Minchat Yitzchak (7:131), who assumes that honorable dayanim will charge only enough to compensate them for the fact that they could not do other work during the court proceedings (s’char batalah), and they will not charge more than the litigants can afford. In addition, they may not accept the testimony of one litigant when his adversary is not present (C.M. 17:5),

 and they must thoroughly investigate all facts (see Rashi’s commentary to Bereishit 11:5). Indeed, the Chazon Ish is often quoted as saying that most erroneous halachic rulings stem from a deficient understanding of the facts.¹²For example, Rav Mordechai Willig (addressing an RCA convention) once recounted how Rav Yonah Reiss traveled to a Midwestern city in order to investigate whether a particular woman had been institutionalized (which might have enabled her husband to receive a document known as a heter me’ah rabbanim). Rav Reiss could have relied on the testimony of local rabbis in that city regarding the woman’s mental state, but he nevertheless traveled there himself, as dayanim must always investigate the facts as thoroughly as possible. Finally, the beit din must not allow rabbis of ordinary stature to rule on matters of great complexity or import (see Teshuvot Meishiv Davar 4:50). For example, the Noda Biy’hudah (vol. 2 Y.D. 88) criticizes an ordinary rabbi for ruling on a case of ro’eh machmat tashmish, a complex area of the laws of family purity that can potentially result in forcing a couple to divorce (see Shulchan Aruch, Yoreh Deah 187). Rav Moshe Feinstein (Teshuvot Igrot Moshe, Even Ha’ezer 1:64) similarly writes that ordinary rabbis should not rule on matters of contraception.¹³