Seajet
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I'd have thought the additional pump the OP refers to would be known as a booster pump, that's what aviators would call it anyway; usually selectable.
What nobody's pointed out so far is that you don't need a back-up!
Lift pumps are generally reliable, and should last a long time, but they certainly can fail - broken rivets, ruptured diaphragm, broken cam-arm - so having a back-up is by no means a waste of time.
But you're presumably talking about a spare pump; other people were talking about a primer bulb