If there's a way to do this in AIM for Windows, have you tried downloading AIM for MacOS from AOL? Normally, I wouldn't see any compelling reason to use it, but in this case....
The only other thing I can think of is to use iChatAV, make sure it's set to put your buddies in your Address Book, and then log in with one, and then the other account, and do the option to add buddies from Address Book?