Triskele,
I think the only coaxes that are uncoupled are the fixed pitch RC toy ones. The gyrodyne had a little tip brake which stuck out on the airstream, which causes aerodynamic drag out at the end of a long lever arm. This requires more torque delivered from the engine and through the transmission. This difference in torque delivered to the rotors yields a net torque at the shaft, and you have yaw moment. Conventional coax helos achieve differential torque by using differential collective. This gives more pitch, more drag, and more thrust from one rotor and less of those three from the other. Same overall thrust but there is an imbalance in torque. The only place this breaks down is in auto. Ray Prouty discussed this phenomenon in a very good article once.
Bryan