Chapter 2, Mishnah 6. This Mishnah continues to discuss the leniencies that the Sages instituted regarding testimony about a woman who was taken captive, establishing that she is permitted to marry a kohen.
The text of the Mishnah:
"Shtei nashim shenishbu" - two women who were taken captive, and there are witnesses to this.
"Zo omeres nishbeisi u'tehorah ani, v'zo omeres nishbeisi u'tehorah ani" - each of them admits that she was taken captive, as the witnesses said, and each claims that she remained pure.
"Einan ne'emanos" - since there are witnesses testifying that they were taken captive, the principle of "the mouth that forbade is the mouth that permitted" does not apply here, and therefore they are not believed when they say about themselves that they are pure.
"U'vizman shehen me'idos zo es zo - harei elu ne'emanos" - when each one testifies about the other that she remained pure, they are believed.
The reason for the ruling:
Regarding the prohibition of a captive woman, as mentioned above, the Sages were more lenient than with other prohibitions. For this reason she is permitted even on the testimony of a single witness, and even on the testimony of a woman, who is generally not qualified to testify, whereas in this case she is qualified to testify.
The concern of "gomlin":
"Gomlin" refers to the concern that the two of them may have made a deal between themselves: I will testify for you and you will testify for me. Even in circumstances where there would seem to be room for such a concern, we do not suspect it, and each one is believed when she says about the other that she remained pure and was not intimate with a gentile - and therefore she is permitted to a kohen.
In summary: Two women who were taken captive, and there are witnesses to their captivity, are not believed when each testifies about herself that she remained pure; but once they testify about one another, they are believed, because of the leniencies that the Sages instituted regarding the prohibition of a captive woman: a single witness is believed, a woman is believed, and we do not even suspect the concern of "gomlin."