I felt they were working out some "plausible deniability" and I agree they have probably done similar many times before. I also got the impression, though, that Pilate in some way wanted to be able to deny it to himself. He was torn between wanting Judas punished and wanting to keep his own hands clean - and I get the feeling he was confused by that himself. I suspect his brush with Jesus had more of an impression on him than even he suspected at that point.