Sounds like you are looking for SSO (single sign on). You have a couple of ways of setting this up:
- Have users authenticate elsewhere (like an existing website), then hook up SSO from both Forums to this site. You’ll have the same user base for both (whoever exists on the parent website), but two distinct forums.
- Make Forum 1 the “parent” forum, which handles authentication, and then act as an SSO provider for Forum 2.
That being said, is there a reason you need to distinct forums? This would be much simpler to have one forum and then create the VIP area via groups and category permissions.