aristoclis - I was thinking of it this way:
Gen 1 and 2 failure... if it didn't happen simultaneously, then the APU will probably already be running following the first failure. If GEN 1 failed and then GEN 2 failed almost immediately afterwards, an APU start is still possible at the expense of battery life.
AC BUS 1 + 2 failure... well, there's no available AC bus to connect the APU to, so why start it?
