It is a cool trick, and very impressive when done well. I think someone mentioned R. Kirk above he is well known for playing five horns at once. I have seen Jeff Coffin (Flecktones) do this and it was awesome. I am currently writing a classical piece for alto and tenor. I was inspired by the aulos (a medieval pipe that has one mouthpiece but two seperate sets of holes so it produces two tones at once). It has been fun working on it and I hope to premiere it at my senior recital in December. Have fun with it. Tracy