The main reason to use counter rotating pros is to eliminate the torque. Your over haul costs double or triple on the counter rotating side, due to the fact that the prop on the counter rotating side are different, as are many of the acessories, such as the vacum pump, Magnetos, and the like. Counter rotating props do not have that much effect in cruise, the big difference between the 2 setups, is in take-off and landing. The Piper Seminle has counter rotating props it is a great plane, and easier to land, and take-off than an Apache, or Aztec, and a Beech 18 is a handful either way. Also there is a BIG difference in "engine out performance". In referance to Fliping the engine over, in a push pull configuration there are a few planes that use that set-up. The Donier 335 was one of them, and the Cessna 337 is a more modern one but the most up to date example is the Adams A-300. For this it is called a center line thrust, if you get your multi engine rating in one of these configurations, your certificate states it and you cant leagaly carry passengers in a conventional multi-engine aircraft. For the simple reason that P-factor and other characteristics of multi engine aircraft are either non existant or nominal. One Problem with a Center line thrust aircraft is that since the thrust line of both engines in in the center line, you do not get the adverse yaw when you loose an engine like you do in a conventional twin. It is harder to determine when you loose an engine on take-off. You still have a "Blue Line" in a push pull but not for the same reasons as in a conventional twin. In a conventional twin the "Blue line" is the minimal single engine controlable airspeed at gross weight. Below the Blue line at gross weight if you loose an engine there is a good chance that you will end up in an aluminum ball somewhere near one end of the ends runway.