Dave,
As a follow on to Nicks comment about past failures (real or mythical).
In the book "Whirlibirds" by Jay P. Spenser, is a photo of the Hiller X-2-235s that had "super-rigid rotors" (coaxial configuration). The photo shows two people standing on the blades, at the tips, with little deflection. I don't know if the machine ever flew, there is mention of vibration problems. Of course, we know that Hiller went with single rotor after his experiments with coaxial. And that seems to be the case with countless pioneers. (such as Sikorsky)
The book is quite interesting for a helo designer.