You can stack domes. Same type repeatedly, different types, whatever.
I would also like to know the exact algorithm for dome activation. I used to assume that each dome would act in turn, as if they were nested, but then I read someplace that only one dome per type can block a single incoming spell, which I guess would mean that stacking forest domes would only increase fire tolerance, while stacking domes of arcane warding would have almost no effect at all....
Well, now I'm subscribed to the thread, so I guess we'll both find out.