one lies and the other swears to it
This phrase is used to respond when one person says something you disbelieve, and then another person comes in to support them. The point is that a lie doesn't become more true simply because another person (of similar sketchiness) tries to back it up.
A: No way man, I wasn't even there that day, you can even ask Jimmy, he'll tell you I wasn't there.
B: Jimmy? Oh, awesome, now Jimmy's backing up your story. Well, just goes to show, "one lies and the other swears to it."
B: Jimmy? Oh, awesome, now Jimmy's backing up your story. Well, just goes to show, "one lies and the other swears to it."