It's been a long time (I'm retired now) but the rotating halves are going at the same speed, they just might not be in phase with each other. A common source was used to flash the fields and the output of the two generators fed a single breaker. You have two freewheeling AC outputs that would naturally sync up on the generator side of the breaker. There's no gearbox so the lagging side could easily come into phase, and then far as the output was concerned it was a single source. After that putting it online was the same as most gen sets, watch the synchroscope and have a sync check relay as safety backup. Although most PLCs do this automatically now, I've done it manually plenty of times.