I think the first model of the P-38 had engines that rotated in the same direction, and it was changed to opposite rotating for the others. The main benefit of having the props rotating in different directions is that the engine's torque balances each other out during the addition of power, alleviating the dangerous torque roll. This would also make rudder input very minimal on take off. As far as engine out conditions, I suppose it could make a difference. Any full scale twin pilots out there wish to comment? I don't think it would make a huge difference with our models. You can modify any engine that has a removable front housing (where the carb is mounted). Remove all the screws, and turn the front housing 90 degrees. Unfortunately, I can't remember which way! This is kind of critical, so hopefully somebody else can answer. Put the screws back in, mount a pusher prop, and you should be good to go. I have never tried this myself, but I have seen one run quite well. I believe it was an older Super Tigre .60.
Good Luck.