A Musikhaus in Germany has been working on a double trumpet (Soprano E[SUP]b[/SUP] and mezzo-soprano B[SUP]b[/SUP]) since the 80's. Still no word. David Monette made a C rotary for Adolph Herseth with the valves about where your design has them. He only made the one. Not to say that you shouldn't undertake the project, heck, you even might come up with something fantastic, but the odds are at best you will get an interesting looking trumpet that maybe kinda sorta plays. At worst, you'll still have a unique instrument that might end up sealed away in a major collection in a hundred years or so.