Hi Rob

It will work as I have myself performed that exact type of conversion.

I basically copied the layout of the rotor system used on AL'S GYRO which used a co-axial setup.
You need to make the blades non flapping with enough of a coning angle to have the same effect as dihedral on a fixed wing.
If memory serves the lower rotor had 7 degrees dihedral and the upper had 14 degrees, mast was tilted aft at 7 degrees . You will probably have to increase the engine downthrust and definitely the model will need a higher hang angle than a conventional flapping head.

I have some images of mine somewhere which I can post here if I can find them.

Best of luck

Sean