Mart,
On the Comanche blade root area were elsatomeric and fluidic dampers, which absorbed the lag oscillations of the blade. The Black hawk and S-76 tail rotors have them, but they are buried inside the cuff and not visible from the outside. Comanche was similar to the EC-135, the brown cylinders on the lower and upper side shown here:
http://212.158.133.3/hwa/antony_morg...135(01)_06.jpg
Dave,
There are some effects that help the power efficiency of a coax, but they are generally cancelled by others that affect power in the wrong direction (swirl effects, extra drag, reduced figure of merit from too much blade area). For the simplistic analysis of disk loading as a hover power determinant, I think things are roughly equal, regardless if there are two disks.
Actually, Igor Sikorsky found that helicopter development was actually set back for decades by those who insisted on the seemingly simpler symmetrical solutions of coaxials, whose mechanical arrangement drove the designs to dizzyingly complexity. Sort of like the old saw, "If its ugly, but it works, its not ugly."