Phil
My concern is the total lack of symmetry at the hub.
A stiff setup leads to big bending forces between an aerodynamically active blade and a basically dead weight that only sees inertial forces. The magnitude of these bending force depend on the target forward speed and the result flap.
One of the nice things about rotor blades is that generally speaking tend to "compensate forces locally", that is aerodynamic and inertial forces, which reduces forces (at least the first order harmonic forces) drastically, and allows for light efficient blades. This also allows as you suggest articulated setups
In order the let the blade go freely one could allow some elasticity between blade and counter weight, but the dynamics of the two are different.
One can easily setup the math model for this, but "ironing out" all the hurdles created by this set up makes me wonder why one would want to do this to begin with: what is wrong with putting two blades.
As Adam suggested the only possible gain could be less blade vortex interaction.
d3