Somewhere on a two-engine setup the torque will be doubled. That is a weakness when two engines are linked to one clutch, primary drive, or transmission and any of the three are designed for one motor. It is easiest for the home builder to engineer and design the final drive. In any case it is a good idea to do some calculations when designing the system. The easiest way is to start with a final drive that is proven to work for one engine and to double the strength for two. This provides some added and probably not needed robustness. Any factor of safety in the single engine final drive is also doubled.
The alternative is to calculate the failure strength and safety factor for the double engine final drive. This is difficult to do. The first method is most practical.