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

I'm going to put my 2 cents in. In a Sling tsi there are 3 pumps, 2 rotax on the firewall and a boost pump in the console forward of the fuel valve. The circuits for both pumps are not the same. I used to turn pump 2 off, but quite often the pressure would drop low enough that the Caution light would turn on and sometimes the engine would stumble. I discovered that the pump 2 switch also controls the boost pump under the console. I now turn off pump 1 while in cruise and use it for a backup. I use pump 2 as the primary (which includes the boost pump) and the pressure never drops very far. I think what's happening is that if you turn pump 2 off then the fuel must be pushed through the non-operating boost pump which creates a great deal of resistance and drops the pressure too far. I don't know about other planes. 

All, regarding pump designs.  The Rotax fuel pumps are a vane pump design.  That said when they are shut off no fuel flows past the pump and that is way they have bypass check valves in the fuel circuit design.  Perhaps Sling can tell you what that 3 pumps design is.  

Cheers