I'm no engineer (as you all remind me), but I grew up around the ship building industry and my professional career was mostly spent around aircraft. The idea of a combining gear box (combining multiple inputs into a single output) is a common concept in ships as well as helicopters. If you think the idea of combining three 235's into a single out put is something, you should see the engine room of a DDG-51 class destroyer (Remember, Bath-built is best built!) -- 4 GE LM2500 gas turbines (26,000 hp each) getting combined into two output shafts (any engine's power can drive either shaft).
Or the combining gearbox on a CH-53K King Stallion -- 3 GE38 gas turbines (~8,000 hp each -- although in tests, the GE38 broke the 10,000 hp water brake ...
) combing to drive a single mainrotor and the tail rotor.
I think the common basic concept is that the input shafts each drive a planetary gear with an overrunning (aka, Sprague) clutch all on a common output. So really doesn't matter if the RPMs are matched or not -- the load/torque will tend to regulate the fastest engine to match the slow pokes and even out the torque inputs. So if you have an issue with an engine and have to drop it off line, it won't kill the others.
Or something like that ...
The ones in ground vehicles, like pulling tractors, etc, probably work on a very similar concept. I *do* know that you do not want to be anywhere near one when they decide to fail -- which usually happens in a fairly spectacular fashion (usually in a high torque mode ...).
