The engine driven generators have an integrated constant speed drive to keep them running at the same speed while the engine speed varies, as the APU is either ON or OFF its generator doesn't need this feature. Dispatch with an engine generator U/S is allowable with the APU running for the entire flight. ETOPS requires a diversion in the event of an IDG failing and being unable to get the APU generator online.
The A320 is much more dependent on electrics and hydraulics then previous generations of conventional aircraft and a higher level of redundancy is called for, two generators offline and I would be looking at suitable enroute alternates rather than continuing a long flight. The emergency electrical configuration is a last resort serious emergency requiring a MAYDAY call, not to be thought of as an additional back up.