At one time I had two submersible pumps in my pit but it was too tight in there for the floats to have room to work. All it would take is for the pumps to move a little. Now I have one pump and it is very well secured so as not to move around and interfere with the float.
What I ended up doing was to put in a high water alarm. If/when it goes off, there is about 4 hours to spare before the water level overflows the pit.
I may at some point in the future install a second pump above the existing one but then I would go with the style that uses probes instead of a float.