Query that marries data between two databases
If you have doubts / questions about your database design, you can improve your question and continue.

Otherwise, let us assume that the database design is perfect - atleast as databases.

You shall start creating views - per requirement - comprising of both databases. And can make all your other select queries to look at the appropriate view. 

They are much faster.

Arriving at a most wanted set of columns of rows matching the where condition through a view is easy.

You shall even think of arranging a view which has everything into it (all rows and all columns - just eliminating duplicates) and use that view in all your queries. This itself will never hinder the performance for sure.