I answered this when you asked on the Rowan/
Kaden Alexander thread, but perhaps you didn't see my reply, so I'll copy it here again:
Well, yes, no and maybe. Incest laws vary from state to state, with each one interpreting the legal meaning of "incest" differently. In some states brother-on-brother isn't illegal and in some it is. So most studios -- in America, anyway -- shy away from that sort of thing to be safe.
I always hoped that we'd see the twins tag-team a third model, taking him for both ends, and am frankly really surprised they haven't done that. That you DO see pretty routinely in studios that have twins.