You might get more traction if this was in the Yamaha outboard section - message a mod and they can move it.
That being said, I don't see anything on Yamaha's website offering dual prop on their outboards. I don't think this is going to be anywhere near as easy as simply dropping the outboard leg and installing a bravo lower unit....going to take $$$$$ in R&D, engineering, fab, etc.
IF it were me, I'd be dumping the boat and buying one with a bravo outdrive as it'd be cheaper.