If you've got a generator problem - start the APU.
If you've got a double AC bus problem - do NOT start the APU.
Not so easy to know which one is faulty.
Both generators, or both AC busses?
Our QRH (320 REV 37) just says: -APU (IF AVAIL) ..................START
They assume the RAT has extended, otherwise APU start will be impossible on batteries only inflight.