Testing and fixture management against two databases

I have some models in my app. that connect to a different database than
the rest of my app.

How do I set things up so that when I run my tests, I can associate the
appropriate fixtures with the appropriate database. For example, I’ll
want most of the fixtures to get loaded into the “test” database, but
I’ll want to load the few that connect to the other DB into another
database (that has a connection name in database.yml).

How is that done?