Question

Hello Rabbi,
In your article on a scriptural decree, you brought the law of conspiring witnesses and the reason the medieval authorities (Rishonim) gave for the uniqueness of conspiring witnesses as opposed to an ordinary contradiction of testimony: in the case of conspiring witnesses, the second pair testifies about the witnesses themselves (the first pair), and therefore they effectively become the accused through the testimony of the second pair (in Nachmanides’ wording: “And the reason is that this testimony concerns the persons of the witnesses, and they are not believed about themselves to say, ‘We did not do this,’ for the others could say about them that they killed a person or desecrated the Sabbath”).
The question is: what would happen in a case where the second pair claims that the first pair really were in that place and at that time, as they claim, but they testify that the first pair could not have seen the murder? For example, they would claim that they (the second pair) (or someone else) blocked the first pair’s view, or that their eyes were covered. According to this reasoning, here too the testimony is about the persons of the witnesses, and seemingly they should be considered conspiring witnesses. But as I understand it, only with the claim “you were with us” do they become conspiring witnesses.