@markushh78 you could simply create a DB connection for your user DB and another connection for everything else and set the User model connection to use the user DB connection.
Use the same user DB connection for each of your sites. You'd have to check that your roles and permission models also use the same connection of course.
@chiefguru Yes this is my current system construct. I already have one user DB. Both sites login can login over this user db. But when i am logged in in site A i am not logged in automatically in site B!.
if it's a login to A and click a button to get to B and vice-versa then you could exchange a token that then logs you in on the other site pushing login stuff into the session.