5
I have not tried by myself but You can use a database trigger to create the users on the second database. I guess it will work on one-way( means you have to keep one database as primary the user has to register only on primary).
Personally, I dont think it's a nice idea to share user database, it may be easier to create one website with 2 user groups. each group will have their own modules to access.
Hope this helps.
Deepak
A member of www.AquariumHomeCare.com