915iS Aux Fuel Pump Operation

All...

We were told by the engineers that the reason to run both pumps is that should your primary fail the engine will stop, instantly I might add.  This engine must have the pressure and without a pump there is never going to be any gravity feed that would keep it running.  In the event you only use one pump then you are faced with an inflight restart.  Given your aircraft design and flight level you then have a lot of other considerations.  As you must, and should always, check both pumps before flight then the safest is to run both in my opinion.  The pumps have to be changed at intervals regardless, I do not know of any pilot that tracks what time is on each single pump.

Cheers