Of the many successfully run multi-engine bikes of the past (and there were a few triple engine ones including a couple of 9 cylinder behemoths , triple-engined two-strokers), the most interesting was John Hobbs' Hobbit.
If you have the opportunity, I implore you to go see it run and talk to John as it is unique in how it is set up.
Most multis used the concept of two identically tuned engines linked together in some way including both chains and gears.
The Hobbit, however, is two twin cylinder engines (one behind the other) set up to run as one four cylinder unit. One main advantage to this was to keep the bike's width to a minimum. Not to mention that Weslake didn't produce a four cylinder engine.
I'm not up to speed with all the technical details of running such a set up (sparking order etc.), but it sure worked well during the time I raced my bikes. The Hobbit was always the one to beat. If you can, take the time to visit with John the next time he runs the beauty then take that opportunity.