From an end user perspective there’s not that much to think about, thankfully.

Basically, it’s like having two websites that mirror each other’s content. You can sign up for Forum A and be able to read and write posts that users on Forum B can also see. People’s names are tagged with the name of the forum they are registered at, but otherwise everything you do and see happens on your own site of choice and there’s no difference where it comes from.

If Forum A doesn’t like Forum C, but Forum B doesn’t mind, Forum A can choose to disconnect from Forum C and hide their users and posts, while Forum B can still see both. It only gets tricky when someone from Forum B makes a post that people from both Forums A and C are in, but all of the posts from C users are invisible to A users.