H-43:
I hear you, but doesn't the trannie have two different outputs, with the associated gearing for the opposite direction drives, extra thrust bearings, output housings, swash plates and collective mechanisms? I assume (could be wrong, won't be the first time! especially since I haven't looked inside one of those trannies ever) that all this stuff is "about equal" to the added burden of a tail gearbox system.
Regarding efficiency, I'll bet you are right, the H-43 is more efficient than an H-1. That has a lot to do with its disk area, which is great, because it was designed in the late 40's for a heavy piston engine, so low low disk loading was needed to get it off the ground. Toss in a turbine, and you have a real screamer. Not necessarily an attribute of its twin rotors, more due to the turbine slipped into the ancient design.
Dave Jackson:
The ABC was a great machine, with its counterrotating system and its very high agility. It weighed about 5% more than a single rotor helo but made that up with excellent maneuverability. Also, it could get to almost 300 MPH (with aux thrust) and pull lots of g at high speed, since retreating blade stall was not an issue.