Simply checking the category boxes performs an OR search; it yields results that have Category A OR Category B. I think the request is for a way to search where every result is of both Category A AND Category B. In this case, anything just in Category A will be excluded from the results.
I noticed in Browse, under the gear icon, there is a filter called "Must be included". I checked to see if this would perform an AND search. Unfortunately it didn't. I assume that it should and that it's broken because when it is the only filter applied it does nothing to the search results. Meaning, it yields the same results as no filters applied. "Must be included" is a powerful feature. I hope it gets working again.
