Two pumps, separate supply electricity if possible. check valves in each discharge into a comon pipe. Just set the float switch on the backup pump to go on at a higher water level. This isn't rocket science, just basic plumbing and electricity.

I have two pumps like this, plus a water-powered jet-like system as a final backup if the electricity and the back-up electricity both fail. Each comes on at a successively higher water level.