Carpass
On most modern aircraft the AC generators are not synchronised. They each power an independant bus, each of which powers independant systems which suffice to operate the aircraft in the event of a bus failure.
When a generator is brought on line, it's respective bus is momentarily unpowered during the transfer.
Typically a two engines aircraft will have a left bus, a right bus and an APU bus with tie breakers isolating them.
When the APU is on line and both engines shut down, both left and right tie breakers are closed allowing the APU to supply both buses.
When the right engine generator is brought on line, the right bus tie breaker opens and the generator breaker closes. Ditto with the left engine.
If a generator fails with the APU off line, both bus tie breakers will close allowing the good generator to supply both buses (with some automatic load shedding.
If a bus suffers a short then the other bus will supply it's own customers in isolation.
Don't know about older types such as the DC10 but I would be surprised if it was much different.
Incidentally, the generators are driven through a constant speed unit which keeps them spinning at a pretty constant speed to maintain 400hz so I doubt changing engine revs would have much effect.
