So first you would need to export your users table (the one with the users) and import it into your friend's user table. You know would have the same users. (You would do this using phpMyAdmin easily)

From here I assume you both make the hacks so that a new user at either board would be entered into the other boards database also.

You would need to give your database info to your friend and he would need yours. The hack would be a minor one to member.php.

You also would need to update a user's info on both boards if/when they choose to change their profile (homepage/password/ICQ/etc)

Again this would be a minor change to member.php.

Remember trying to maintain synchronized data on separate databases is not a fun task. What happens if a user regs at your server but your friend's server is down or inaccessible at that moment? You are now out of sync. You could write even more code too make sure a queue is maintained so that the database's can be synched but the hack then becomes tedious. What happens when the user's userids get out of sync? Userid is an auto-increment field and can easily get out of whack. User A regs at your board and user B regs at your friends board at the exact same moment and they both get userid 40. User A then get 41 on your friends board and User B gets 41 on your board. You now see they are different. vB operates off of the $bbuserid so it would be important for them to match. I hope I have scared you enough to not try this. I would suggest you just make (or ask) for a hack that would modify only one of your boards (yours or your friends) to use the other boards user database. You would have no synchronization problems with that outcome.