When 2 potential witnesses deny any knowledge of the event(s) they're asked to testify about, the claim is that they need to deny at exactly the same time. And since that precise of a time is not possible, perhaps they weren't really liable... (this is the view of R. Yosi HaGlili, who says they could have spoken at "the same time," with a certain amount of time to make a short utterance. Also, the case of one witness -- when that would be acceptable, and when insufficient. Note the eye to the protection of women.