a counter rotating of the tractor and the pusher tend to even out the swirl. Yet efficiency goes down the drain because a narrower column of air is accelerated, much increasing the apparent prop disk load. It was no success in German WW2 fighters, but that is no reason not to do it in a model airplane.
After a quick failure mode analisys I think, the Cox pusher needs a hardened steel or phosphorbronze shim between the prop drive washer and the crankcase.