This is just my slightly educated guess, but...
I'd guess that if the generators are inverter type generators and they get unlinked, you'd get some weird waveform instead of a nice 60 hz (+/-) sine wave to your loads. Hard to predict what the exact effect would be, but not good.
If it's a conventional generator and you lose the synch link once running , I'd guess you'd be okay, although you may not get the full dual/equal outputs. But I'd guess everything would still be in synch. Though one generator may generating and the other is motoring (but more likely somewhere in between, the governor, throttle positions, engine output, etc.. come into play). But note, even a generator and a motor are still (relatively) "in synch" with the common sine wave being produced/consumed.
The real problem is you have one generator running, making 60 hz, and then you "switch in" another generator that is also running, making 60 hz, but their sine waves are 180 degrees out of synch. They are now trying to instantaneously spin the others rotor 180 degrees. But then again, this is like starting a motor and going from 0-60hz.
It's likely that on small induction (slip) type, low flywheel mass, with generators of equal size, they'd be little damage as the two fight each other to arrive at synchronicity. Probably same if you start one after the other is running.
It becomes a real problem when you do this at a power plant with a generator where the rotor has tons on mass, and the you hook it into "the grid" with mega-watts of power that attempts to spin tons of generator's rotor 180 degrees instantly. This is when you get a loud bang. Rotors snap, bearings fail and 1500 tons of generator come flying up through the floor.
