I'd mount that "second identical one" in the bilge and wire it into another float switch or a water witch style switch,I don't have much faith in the self contained
ones with built in float, Like already mentioned it doesnt take much to jam up a float, more of a worry if your moored rather then on the trailer, google water witch its pretty cool no moving parts to jam and easy install, you can even add a 3 way switch if you want so you can have manual and auto modes if your not always with the boat
then go buy another one just to keep for back-up.
I run 2 pumps, I know some guys run 3 or more