Gaseous,
Why are there no 2 blade fully articulated heads ...
There would be a 2/rev vibration when the tip-path plane was not aligned with the mast plane.
212man,
I'm not sure why you desire this propeller like rigidity? .........
If a compound helicopter was to take its rotor and wings, and combine them into a pair of rotors with "propeller like rigidity", we would be looking at the next generation of rotorcraft.
A brief look at history.
Cierva's early gyrocopter had a rigid rotor. When he tried to takeoff, the dissymmetry of lift rolled the craft onto its side. He then went to his English financial backers and attempted to get more money to add a second rotor. He knew that the two latterly located rigid rotors would give the craft symmetry of lift. His backers went ballistic. They started flapping their arms at the prospect of putting more money into his crazy idea. Cierva's English was very poor. He therefor though that they wanted flapping hinges on the existing single rotor.
Well the rest is history. For the next eighty years, Rotorland has been flapping its gums and spinning its wheels trying to make the single rotor a success. 
Dave