Bad idea to use multiple databases?
 

I was just wondering if there were any disadvantages to using multiple databases to run a website in PHP and MySQL. Are there any performance differences significant enough to take note of when switching between databases?

Oh and also--would it be a problem to run two websites off of a central "users" database... That's really what I'm getting at. Of course these websites would be either on the same machine or behind the same hub.

At the moment i'm not concerned with the bandwidth efficiency but moreso the functionality and if this will cause any performance problems for either website and what the disadvantages are of a design like this.