Rif Ketubot 10b {Ketubot 23b} Two Captive Woman Testifying About Each Other; And So For Kohanim

10b

{Ketubot 23b}
Mishna:

שתי נשים שנשבו זאת אומרת נשביתי וטהורה אני וזאת אומרת נשביתי וטהורה אני אינן נאמנות ובזמן שהן מעידות זו על זו נאמנות
וכן שני אנשים זה אומר כהן אני וזה אומר כהן אני אין נאמנין ואם העידו זה על זה הרי אלו נאמנין

Two women who were taken captive -- this one says "I was taken captive and am pure," and this one says "I was taken captive and am pure," they are not believed. But when they testify each on the other, they are believed.
And so too two men, this one says "I am a kohen" and this one says "I am a kohen," they are not believed. But if they testify one about the other, they are believed.