The Help:Category article says:
An "OR" can be added to join the contents of one category with the contents of another. For example, enterincategory:"Suspension bridges" OR incategory:"Bridges in New York City"
to return all pages that belong to either (or both) of the categories, as here.
But this search returns no results even though each category by itself returns many results. I've tried this with other categories and it appears that it does an AND (intersection) regardless of the actual operator used.