> once beis din determines that both groups are equally trustworthy the chidush is we beelive the second pair
Maybe a following sevorah: the first pair appeared to testify on their own. They had a chance to fit their testimony to the person and time of their choosing. They had time to consider all circumstances and, thus, it is pretty possible, they had a chance to frame someone. After this event is fixed, now second pair needs to address two specific people at a given time. The time that they have a reason and a possibility to frame them is much lower.