Aux Pump

Based upon the answer I got to this question at the Rotax maintenance and repair course, starting on one fuel pump is to minimize battery voltage drop with starter inrush on a weak battery.

You can start with One Pump or two.
The Pumps are in Series.
With Both Pumps ON, in theory, the Flow Rate and Fuel Pressure are unchanged.
In reality, there will be a very slightly increased.
Not enough to make any difference, but the increased power draw might make a difference if the battery is marginal.

You can also start on Lane B only with no consequences.
The engine ALWAYS Starts on lane B alone and switches to Lane A once the RPMs reach 2400+ for a few seconds.
If you shut OFF the start power before activating the A Lane, you will note that the Battery is NOT charging until Lane A activates and the B Generator is released to power the panel and charge the battery.

The 912is engine is NEVER going to start after one blade.
The ECU requires monitoring the Crank-Sensor for two full revolutions at cranking speed before acquiring Crankshaft synchronization and activation of the ignition coils.
You will always hear at least 4 compressions before the engine starts.
Don't get impatient and think it isn't going to start.  Turn the Key and hold it.

After starting, monitor your battery voltage.
Rev up to 2400+ and wait for the Battery Voltage to jump above 13+ volts suddenly.
After that, you can reduce to full idle.  Lane A is now online.

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Rotax call for Dual Fuel Pump Power Switches.
Both Pumps are equal.  Main and Aux are arbitrary designations.
Part of the preflight operational checks includes verifying each LANE Independently. (Mag Check)
And then Verifying Each Fuel Pump Independently. (Pump Check)
You can't truly verify the AUX Pump's operation if you can't disable the MAIN Pump.

It is good practice to operate BOTH Pumps at or below pattern Altitude.
Once in Cruise, One or the other pump can be disabled to extend their lifespans.

Sounds like the manufacturer's POH is barely adequate. Here is the Vans RV-12iS POH which I hope will help you operate your 912iS equipped aircraft better.