Two Domain Names, One Database

I currently have one membership website with chat rooms, forums, directory, etc. I’m running into problems with too many plugins, conflicts, speed issues, etc.

I want to put the chat rooms on a separate domain name, but make that second domain name a link from the main site so that my members only have to login once to access all features.

How do I use the database from the original site on the second site to allow my members to use both sites with the same profile?

Thank you.