How to implement a "subordinate database"?

I keep bumping against this situation: I have a main database A, and I want to implement a database B, that is distinct from A, but subordinate to it, meaning that it refers to data in A, but not vice versa.

 

I don't simply want to add new tables to A to implement B, because this unnecessarily clutters A's schema with tables that entirely extraneous to it.

 

Is there some other way?

 

Thanks!

 

kj