If I understand this correct you cannot expand the search beyond one hash tag, correct? You can only get more specific. So for example, right now I want to search for #tag1 or #tag2, whereas if I enter #tag1 & #tag2 both those tags will have to be present in a tweet for it to show up.
If there is a way to do this, please let me know, if not then this would be an important feature request.