I am currently working through my head an idea for a 3.5e DND campaign I want to DM, and as part of that I want to have various different people knowing different parts of the map depending on stats/background etc.

With 1 person I see it being easy, just add a layer with everything they don't know fogged out. Same with person 2.

But then I want to be able to see the area the group know by turning both layers on, but this will give me the area both know, rather than the area either know.

Is there a way to make it so the clear areas not the shaded areas stack?

(Software is GIMP)