If you use different size props on the front and back props, I guess you could use a smaller engine in the back. I know that the 234 used the same size props, so I presumed that you wanted to do the same on your model. The problem will arise though, because you will be using such large props ,much larger than on a Saito, that the one engine will cause undue drag for the other engine and will result in reduced performance for both. A Saito turns a much smaller prop with a much lighter pitch and this fact is nowhere as evident and critical.
When I spoke of a fan for this application, I was speaking of using a small electric motor like a Graupner Speed 280 turning a small fan blade to move air threw the upper cooling scoop. The rear engine will have to be baffled very carefully to provide the right cooling air flow. I would study the air flow baffles on a full size radial air cooled engine for this. To see how the air flow from one cylinder is prevented from flowing over the cylinder behind it. The front engine will be exposed to airflow at all times due to the radial opening in the front of the aircraft, and the prop directly blowing air threw it. You will have to create a rear exit hole for this air, due to the fact that the cowl flaps on the aircraft are much too far forward. The cowl flaps were there to accommodate a radiator in front of the engine, not to accommodate an air cooled engine, but the radiator opening lends itself nicely to the installation of the RCV engine. You will also have to create a baffle to flow the air fro top to bottom. If this is not done than the fire wall will prevent the engine from cooling correctly also.

Thank You
Otto Kudrna