Bava Batra 32A

ורבן שמעון בן גמליאל סבר: ערער ⁠– תרי; והאמר רבי יוחנן: דברי הכל אין ערער פחות משנים!

and Rabban Shimon Ben Gamliel holds that an effective challenge requires two witnesses, one could then ask: But doesn’t R' Yoḥanan say: Everyone agrees that there is no effective challenge with fewer than two witnesses?

אלא ערער תרי; והכא במאי עסקינן ⁠– כגון דמחזקינן ליה באבוה דהאי ⁠– דכהן הוא; ונפק עליה קלא דבן גרושה ובן חלוצה הוא, ואחתיניה; ואתא עד אחד ואמר דכהן הוא, ואסקיניה;

Rather, the challenge was established by two witnesses. And with what are we dealing here? We are dealing with a case where we presume with regard to the father of that man that he is a priest, and a rumor emerged about the son that he is the son of a priest and a divorced woman, or the son of a priest and a ḥalutza, and we downgraded him from the presumptive status of priesthood based on that rumor, and one witness came and said that the man in question is a priest of unflawed lineage, and we elevated him back to the priesthood, as one witness is sufficient to negate a rumor.

ואתו בי תרי ואמרי דבן גרושה וחלוצה הוא, ואחתיניה; ואתא עד אחד ואמר דכהן הוא. ודכולי עלמא ⁠– מצטרפין עדות,

The Talmud continues the case: And then two witnesses came and said that he is the son of a divorced woman or the son of a ḥalutza, and we downgraded him from the priesthood based on their testimony. Then one witness came and said that he is a priest of unflawed lineage, resulting in two witnesses testifying that his lineage is unflawed, and two testifying that it is flawed. And everyone agrees that the testimony of the two single witnesses combine to produce testimony that he is a priest of unflawed lineage, and his presumptive status of priesthood should be restored.

והכא במיחש לזילותא דבי דינא קא מיפלגי ⁠– רבי אלעזר סבר: כיון דאחתיניה ⁠– לא מסקינן ליה, חיישינן לזילותא דבי דינא. ורבן שמעון בן גמליאל סבר: אנן אחתיניה, ואנן מסקינן ליה; ולזילותא דבי דינא לא חיישינן.

The Talmud explains the dispute: And here it is with regard to concern about contempt of court that they disagree. R' Elazar holds: Once we downgraded him from the presumptive status of priesthood based on the testimony of two witnesses, we do not then elevate him, as we are concerned about contempt of court, as a reversal in the court’s decision creates the impression that the court operates indecisively. And Rabban Shimon Ben Gamliel holds: We downgraded him from the presumptive status of priesthood and we then elevate him, and we are not concerned about contempt of court. The primary concern is that the matter should be determined based on the relevant testimonies.