Here is what we wrote in our Background to the Daf (Sanhedrin 41):

EDUS SHE'IY ATAH YACHOL L'HAZIMAH, LO SHMAH EDUS - Testimony that Cannot be Contradicted is Not Valid Testimony

(a) When testifying in Beis Din, witnesses must clearly state both when and where they observed the event that they claim to have witnessed. If another set of witnesses subsequently claims that the first set were elsewhere at the time that they claimed to have witnessed the event, then the first set are termed Edim Zomemin (see above, entry #11). Testimony that cannot be contradicted in this way is not acceptable in Beis Din.

(b) Our Gemara asks that if the witnesses are able to evade punishment with the claim that they intended only to prohibit her to her husband, how can she be killed on the basis of such testimony even if she is an Ishah Chaveirah? Since they cannot receive the same punishment that she would have receive had their testimony been accepted, their testimony should not have been acceptable in the first place!

Kollel Iyun Hadaf