How to display results of multiple search criteria

Anyone know how to do this? I want to be able to display posts that meet multiple category criteria. For instance, let's say you have ten posts in the "Art" category and ten in the "San Francisco" category, but only 3 are in both. I want to be able to display only those three.

Is there a way to submit that kind of search criteria?

I know this isn't Atahualpa specific but I was hoping that some clever person here might have an answer one way or the other.

Thanks!!
Sharon