Hmmm, not once Rich? I can't believe it hasn't been tried yet. Think of the soloing options!!!!
You could just use 2- 9's for the top! You wouldn't need to go thinner would you? I think it would work, and you could make the jump to 7 easier. I'd think it would also be easier to mute the top than the bottom and you keep all the chord patterns the same. Now I'm excited