Just a thought
I had a similar idea to Daves without the gears. When I worked for Erickson we had a test cell that would run one engine and one main gearbox at the same time. Power that was being absorbed by the cell was redirected to the input of the gearbox that was not occupied by the engine. Why not use this for yaw control on a coaxial? I was thinking more along the lines of a motor/generator on each mast. When you apply a field current to one as a generator it will create a drag on that rotor mast. The energy absorbed by that generator would be sent to the motor on the opposite mast to recoup the lost power. In an of itself it me not be perfect as the motor/generators would obviously add weight and complexity it could also be useful as a hybrid drive of sorts. During cruise flight equal current could be drawn from both systems to charge a battery pack which could be used as additional boost of power in the case of engine failure. You could even use it for normal operations and scale down the engine such that a max power take off may require both engine and battery power and then the smaller engine package is left to carry the load during cruise. Of course that wouldnt be such a great thing for an aircraft destined for long hover times as sooner or later you are going to run out of battery power!!
Like I said,...just a thought
Max