My question would be "How will you ever know if one pump quits working?" Without some way of knowing this, or of testing each pump separately, one pump might fail, and you would have no way to know you were operating with no redundancy until the second one failed. You'd be no better off than if you only had one pump!
Personal experience: I was flying with a buddy who had an engine-driven fuel pump fail enroute on a cross-country (Lycoming engine, not Rotax). My friend had forgotten to do the "cruise" checklist when we first got to our cruising altitude, but remembered to do it 30 minutes later. Most of the stuff on the checklist had already been done, except for turning the fuel pump "off"... He turned the switch off, and 5-6 seconds later, the engine quit. Clearly, the engine-driven fuel pump had failed sometime after we did our run-up check pre-takeoff, and we had no indication of that fact until the electric pump was turned off and the fuel stopped flowing!
Of course, he turned the electric fuel pump back on, and within 3-5 seconds the engine was purring along again, as though nothing had happened.
As we flew on to the nearest airport with a mechanic available, we discussed how the checklist procedures ensured both pumps were working before you departed. In his airplane's case, the electric fuel pump was turned ON before starting the engine, so you could observe a rise in fuel pressure (and hear the pump, by the way), then turned OFF before actually starting the engine, which would verify that the engine-driven pump was working.
We also discussed what a great idea it was for the checklist to require the electric pump to be turned ON as part of the before-takeoff actions, since we had no way of knowing exactly when the engine-driven pump had failed. Worst-case scenario could have been below 500 ft on takeoff, or while we were at 1000 ft above a densely populated area.
Making a long answer shorter, I would NOT operate with both pumps on all the time, unless there is an indication for the pilot to know if one or the other fails. From what I understand, the Facet pumps we normally use for Rotax engines don't have any "internal check" for status, and could well continue to draw power even when the pump quits working.
