joining separate databases

Hello!

This may be missing the mark completely, but I figured I would ask:

I have a database containing our customers and our information product, and
I'm working on an internal project management app that will be stored in
another database (different machine, even).

Now, it occurs to me that I will want to join some tables from one database
with some tables in another one in-a-while, for instance if a customer is
responsible for a project, their contact information should be pulled from
the first database.

Or, to put it another way, I have two completely different problem domains
that intersect. I have a database for each domain. What if I want to join
them in some way? It just doesn't seem right to put every single table my
company creates into the same database, but I may be wrong.

Can anyone tell me where I'm going with this? Is there a school of thought,
or best practices, that I'm looking for? Anyone have a subject that can be
searched for?

Thanks!

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