One of the primary reasons of having a pushme pullyou configuration is that the torque from each motor cancels out the torque from the other making for much better handling qualities. If the rear motor is reversed (and a tractor prop used) the torque will be additive resulting in an increased yaw tendancy to the left. Since you don't have a rudder in the slipstream, it may cause handling problems at low speeds. Pusher props are very much available from Zinger, Master airscrew, APC and Graupner. Pusher props for electrics, thought, are pretty rare..... but there is law saying that yoiu have to use electric props...FWIW.