915iS Aux Fuel Pump Operation

This is more of a function of personal philosophy than anything else. The "primary" and "aux" pumps are identical. One could just as easily have labeled them pump 1 and pump 2. (And some have.) That's why there's negligible difference in pressure with one or both running: they're two identical pumps running in parallel. 

Some say to turn off one pump in cruise to save wear & tear on it. Some pilots always turn off the "aux" or pump 2, others alternate: turn off pump 1 one flight, turn off pump 2 the next flight.

Others (knowing that if you're running with one pump, if that pump fails your engine would fail) operate with both pumps all of the time. In a recent webinar on EAA.org, a Rotax expert recommended this procedure, especially if running mogas in the US.

I started off in the former camp; I would turn one pump off in cruise. Now I have changed to the latter camp: I leave them both on. I check them individually before and after each flight, but I leave them both on at all times in flight. 

Anyway, that's my 2 cents. I'm open to other opinions.

Bottom line is running both pumps detrimental to the engine? No

If Rotax thought that it was absolutely necessary to run both pumps all the time don't you think they'd say something. The 912iS is in the same boat. 

Rotax is good about keeping people informed especially compared to other engine MFG's and aircraft Mfg's.

The pilots I know just use one pump in flight and don't use the 2nd pump all the time so as to keep it for a backup like it was designed for. The fuel pressure regulator controls your pressure. Normally you may see zero to maybe 1 psi difference. Fuel pressure is well controlled. Not sure I see the logic in running both pumps solely because of Mogas. You either have good fuel pressure or you don't. If you're flying and you see an issue with fuel pressure then use the aux. pump. I have friends that have flown from Arizona to Alaska 2-3 times, across Canada, all around the US, through Mexico and the Bahamas and never use both pumps just for normal cruise. During trips you're kind of stuck using 100LL, but many of the guys I know that fly around their general area use 91 Oct.

The recommendation of running both pumps with mogas came from Rotax Flying Safety Club instructor Jorge Tavio, so you can ask him. Many have had occasions with mogas where one pump would not provide sufficient fuel pressure, but two would. So rather than having your engine quit first and then select the second pump, the suggestion is to just run them both and prevent all that consternation.

Interesting answers.  Since I have to run 100LL guess I will migrate to the two pump camp.