Related for Ketubot 2:8
רַבִּי יְהוּדָה אוֹמֵר, אֵין מַעֲלִין לַכְּהֻנָּה עַל פִּי עֵד אֶחָד. אָמַר רַבִּי אֶלְעָזָר, אֵימָתַי, בִּמְקוֹם שֶׁיֵּשׁ עוֹרְרִין. אֲבָל בִּמְקוֹם שֶׁאֵין עוֹרְרִין, מַעֲלִין לַכְּהֻנָּה עַל פִּי עֵד אֶחָד. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר מִשּׁוּם רַבִּי שִׁמְעוֹן בֶּן הַסְּגָן, מַעֲלִין לַכְּהֻנָּה עַל פִּי עֵד אֶחָד:
R. Yehudah says: One is not elevated to the priesthood by the testimony of one witness. [Even if there are no "reciprocal" witnesses, and, it goes without saying, if there is a possibility of reciprocity, i.e., Testify for me, and I will testify for you.] R. Elazar said: When is this so? Where there are "objectors" [who declare him to be unfit for the priesthood; and there is no "objection" with fewer than two], but where there are no objectors, one is elevated to the priesthood by the testimony of one witness [where there are no reciprocal witnesses. And this is the difference between R. Elazar and the first tanna (R. Yehudah)]. R. Shimon b. Gamliel says in the name of R. Shimon the son of the sagan (the adjutant high-priest): One is elevated to the priesthood by the testimony of one witness. [The gemara asks: "Aren't R. Shimon b. Gamliel and R. Elazar saying the same thing!" And it concludes that they differ on the question of "combining testimony" — as when we know that this man's father was held to be a (fit) Cohein, and a report went out that he was the son of a divorcée or of a chalutzah, and he was "taken down" (from the priesthood), and then a witness came and said: I know him to be a (fit) Cohein — at which he was re-elevated; and then two witnesses came and said: He is the son of a divorcée or of a chalutzah — at which he was again taken down; and one witness came and said: I know him to be a (fit) Cohein — R. Shimon b. Gamliel says that he is re-elevated to the priesthood by the testimony of this last witness, for we "combine" him with the first witness who nullified the report by saying: I know him to be a Cohein. And even though they did not testify at the same time, their testimony is combined, and we say: Set these two who say that he is a Cohein beside the two who say that he is the son of a divorcée, and "set the man into his (original) status" (as a fit priest). And according to R. Elazar, he is not re-elevated until two witnesses testify at the same time that he is a (fit) priest. The halachah is in accordance with R. Shimon b. Gamliel, that the witnesses are combined even though they did not testify together.]