I am interested in this as well and I have just been having a wee look about and trying to work out how i could do this.
First off I am not the best php programmer but i get by and I am new to wordpress and just finding out how it all works.
So here are my thoughts on what i could do.
1.integrate the two user tables in the DB to one. The vanilla one has more fields so use that. Now to do this i would change the name of the table in wp-settings.php (easy) then go though the files that insert into the table and change SQL query’s or the forms.
Problems – I dont know if its a good idea going in changing stuff in wordpress in case i cant get new plugins working in the future.
2.Keep two tables for different users and run some sort of script after each person registers that inserts the user into the other DB. Also when someone logs in run some script that logs them into the other.
I dont like the thought of two tables for users seems just bad.
I dont have the first clue how to do this in a plug in so any advice or plugins already built that do something similar would be very helpfully.
any thoughts, even if its pointing out how my ideas would never work would be gratefully.