Since most people's speculation on these cases are based on TV shows like Law and Order, I'm going to indulge -- one possible scenario is the trick where our hero detectives know that the two guys are involved, but couldn't get anything out of them in questioning. So they release them both to see if the two guys get together afterwards and blab (if they really are guilty, it's still an open question whether they'll have the sense to avoid using the phone). Or -- one guy says he's the alibi, but the detectives say "we'll drop the charge of lying to the police if you get the other guy on tape saying he did it."