Very few real life full size twin engine planes or multi-engine planes used contra-rotating props and or engines. It caused a parts support nightmare for the maintenance people. Many of the ones that did quit doing it too.

But for twin engine RC planes, I simply mount both normal rotating engines like normal. usually both engines are side mounted with the engine cylinder both facing to the right. it has not been a problem. The planes fly just fine like that. Keeping it simple simplifies the entire process. With gas engines most guys mount them inverted, and it isn't a problem for them either. With my smaller glow engine powered twin engine planes, I treat a engine flame out like a dead stick scenario and I cut the throttle and land the plane right away. So I haven't crashed a twin in quite a while due to a engine flame out. I also adjust each engine to run good. I don't try to synchronize the engines. The plane flies fine even is one engine is off quite a bit from the other. The engines don't have to match up with each other. I know guys flying twins and four and five engines planes using different engines and the planes still flew just fine like that.

Now as the plane gets larger and larger, the rudder become more and more important. You need to make coordinated rudder and aileron movements to make the turns. Using ailerons by themselves will likely not work on the big RC planes as they now fly more like the real planes do. So you need to get good doing coordinated rudder aileron turns. usually the larger planes have a fairly large amount of dihedral in the wings and that reduces the effectiveness of the ailerons, but it increases the effectiveness of the rudder though.

Anyway, to reverse a gas engine, one needs to relocate the timing sensor to the other side so it is opposite of where it currently is located. You can use a degree wheel disc to match it back up. You don't have to reverse the cylinder unless you want to, but if you do, then you need to reverse the piston too.