hdzz_top you can store multiple independent Flarum installations in the same database with different table prefixes, but you rarely need to do that unless you are using a very restrictive hosting that doesn't let you create more databases.
Flarum has no support for "federated" websites or "sub-communities" out of the box. That's something that you would have to write yourself.
If you're just talking about a single forum with multiple public domains, this is not recommended for SEO purposes. It's better to redirect all alternate URLs to one canonical URL.
