No one makes a counter rotating prop setup that I know of. I have been playing around with the idea but you are going to add a lot of weight to a model with the gearbox. You will need a right and left hand prop along with a gear box to drive the props and a motor with enough power to drive two props of the size you want. The gear box would be similar to what is drawn with a shaft right through the the box attached to the gear on input side and gear on output side drives counter rotating prop. The yellow in the drawing is bearing material and the pink is the outer case. Built one in the 80's but would only spin 10" props with a 90 size engine for power. Modern engines are more powerful but still not realy up to spec.

Cheers.