Anyone can help me ? Two licenses, two different domain, but I want a common login

[Reeve of shinra]

For an existing board, you may have to import the users from the slave forum to the master forum ... then manually run some queries in the slave forum to update the user id's in the threads and stuff.

As for the mod, the mod might work up to 3.8 with little change.

I personally think Adrian has the better approach with using mysql views as it doesn't touch the code, just the db.