Thank you for all your replies.
I have some things unclear though - shouldn't the rear prop be smaller in diameter and higher in pitch? If the first prop already accelerates the air, it would be normal to have the second propeller move at higher RPM that the first one to accelerate the air even more right?
Since the engines are cox td 09, i've heard that the best propeller for that is a 7x3.5 (i already have a 7x3), so I am thinking to get a 6x4 pusher propeller for the rear engine.
As for the reinforcement between the prop drive washer and the crankcase for the rear engine, does the incoming air create that much pressure to affect the crankshaft of the rear engine?
thanks.