Why not just have three db's. One central user database containing the user, userprofile tables etc and then another two for the rest (e.g. forums, settings, templates etc). Then have all 3 database all on the same username and password. Then do a long-haul search and replace within the file replacing all references to the " . TABLE_PREFIX ."user table for example with nameofuserdb." . TABLE_PREFIX . "user
That'd be quite easy to do