Not a design/certification requirement.
APU can have MEL applied so Murphy's Law dictates that your fuel exhaustion event will coincide with your APU being unserviceable.
With no noise from the main engines, time from max cruise ALT to granite is approx 20 minutes in a commercial airliner - why the need for 2 hours APU ops?
If you want an extra 2 hours for APU ops, then why not mandate taking an extra 2 hours fuel for all your main engines?
Standby power/batteries/RAT/static inverter is a well designed and certified system for the fuel exhaustion/loss of elec generators scenario.