Main Report from DB A and Subreport from DB B

I have a main report that gets the count of ProviderID's from DB A based on a parameter and a subreport that uses the same parameter that gets the count of ProviderID's from DB B. Both databases are on the same server.

The last requirement is a subreport (count) of the number ProviderID's that match in both databases. I was hoping to use a shared variable, but that's not working. I'm pretty illiterate when it comes to writing an SQL statement, so I was hoping there was another workaround. From what I'v read on the forum a UNION statement(?) is required if the db's are on separate servers, but these are on the same server and I'm using them in one CR report.

Any help would be invaluable.