Add selection as add and intersect commands combined

Hello, I want to add selection to a selection so it acts as the two commands combined: add command and intersect command. Where parts of a selection 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. Can this be scripted?