A lot depends on how you are opening your second db. You need to start another instance of Access. If you go to the file menu or use a standard commwnd within an already open db then, by default, Access will switch to the new db rather than open another instance. I believe there is a way to change this behaviour but I don't know how and I can't find it anywhere.
If you are using a command button to open another db then include the start up instructions for opening another instance of the application rather than just telling Access to open another db.
Otherwise create a shortcut to open your db directly as another instance of Access.