Yeah, only one credential per database. It actually doesn't matter if its a different user or not. In the Ruby driver we require the first user to manually logout of that database before another user can login on the same client instance. 

I suppose you could go the route of trying to detect if the user is different and handle the logout for them, but in our case we decided that would introduce unnecessary complication into the auth flow. We wanted to keep it simple and explicit, with little or no magic happening behind the scenes.