Shevuos 4:3-4

Shevuos 4:3

An oath of testimony is as follows: a litigant in a court case asks two people to come testify for him and they reply by swearing that they have no information that would benefit his case, or they say that they have no useful information and he imposes an oath upon them, which they confirm by responding “amen.” In such a case, they are liable (assuming that they lied). If he imposed an oath upon them five times outside of court, then they came to court and admitted having information, they are exempt; if they denied it in court, they are liable for each oath. If he imposed the oath on them five times in court and then they denied it, they are only liable once. Rabbi Shimon said that the reason for this is because, once their statement has been given, they cannot retract it.

Shevuos 4:4

If the two potential witnesses denied having information together, they are both liable. If one of them denied after the other, the first person is liable and the second is exempt (because his testimony would be ineffective without the other witness anyway). If one of them denied having information but the other admitted it, then only the one who denied it is liable. If there were two groups of witnesses and one denied having information, then the other denied having information, both groups are liable because either group could have validated the litigant’s testimony.