^ How about Bert and Ernie? If that doesn't float your boat (sorry for the pun) how about Batman and Robin (or The Joker) or Harry Potter and Dumbledore or a couple of nuns or Caesar and Brutus.
If you don't insist on going dresses as a pair, how about doing classic horror characters like Frankenstein and Dracula or the Mummy and Wolfman. And speaking of classic, how about characters from literature. One could do King Arthur and the other could go as Jay Gatsby or something like that.