Add and Intersect commands combined

I want to add selection to a selection so it acts as the two commands combined: add command and intersect command. Where parts of the two selections intersect i want to act it as the add command - it simply adds selection to a selection. Where it does not intersect, i want it to act as the intersect command - it deletes the part of the selection that doesnt intersect. It seems like photoshop is incapable of doing it...