So that when you login on one of them, then go the other, you're logged on that one too. I know i need something like a migration script?! but can't find an example to modify it for my needs. The databases are not identical.
do you want it to open the window for the other site as well? if not share the logins or database user account between the 2 sites, and then pass the session variables, or login cookies, sessions for the other site to store for say 24 hours so that if they go there they are auto logged in based on details given in other site.