registration into category user tables ?

while reading the forum threads a thought came to mind...

what if at registration time you could allow the members to choose a 'user table' group to belong to ?

right now there's one user table for all members. but what if you say had 2 user tables , user1 and user2 tables

so at sign up time the user could select in a text box, user1 or user2 group and have their member details added to user1 or user2 table ?

or maybe have 2 sign up forms for 2 separate user1/user2 tables

it would be great to have 2 member bases for 2 different site themes on the 1 forum... it ain't the cost that's my problem it's the time to manage and maintain a forum...

but would this place a very increased load on mysql ?