You certainly can, but it's not easy to do, so we don't provide our own guide.
Here's a StackExchange question that should help:
Is federation a topic that has been explored?
I'm interested from the perspective of loosely affiliated online communities.
Please share your own thoughts. I have seen your forum and read a topic about being happy when using NodeBB.
Do you mean SSO across multiple forums? It's something that we can do, but we wouldn't bother unless a lot of forums were interested first
I think this feature is called Multisite (i.e. User Database Sharing).