Multiple Installations, 1 Database but multiple table prefixes

I would like the user table to be managed by all the sites so I named it userwp_users and userwp_usermeta instead of the default wp_users and wp_usermeta.

How do I connect the wp-settings.php to the user table alone?

And is it possible for the other tables such as the posts etc to be managed by separate installations of wordpress?

All of the other sites will have separate installations of wordpress on it but their own parent theme and functions.

Will I have to let them manage separate table prefixes?

Thanks