If you've only got one crib for now, it's helpful to also have a pack'n'play handy so that if you need to separate them, you've got something available. If you have room for two cribs, I've heard some families set both up even if their babies are sharing a crib because if you have late night spitup or diaper leaks, you can just move the babies to the other crib while you deal with clean up.
My 9YO twins have regular twin beds (DH's & my old ones from our single days) - we considered bunk beds but decided against it b/c one of them didn't want bunk beds, and also we were concerned that the one with the top bunk would be too warm in the summer time.