I would say let them sniff each other first, while holding one or both of them, if that goes well, put them somewhere together and see how they react to each other. If that goes well, you can probably just put them together (after cleaning everything like Capn said, so that everything is neutral). Rats usually get along pretty well, so it should be pretty easy. When you're introducing them and when/if they go together, keep a very close eye on them for a couple days. Good luck, I hope they get along!