Bay Rat......Aquarius gave you the best advice, two separate pumps with two independent float switches.
There happens to be a good little article on a new system if you are looking to stick to using one bilge pump:
New approach to the bilge switch
A simple but effective way to CORRECTLY rig a automatic bilge pump:
Installing a Bilge Pump
A excellent review on the various bilge pumps on the market:
20 Electric Bilge Pumps Tested
One rule of thumb that the article clearly outlined with choosing a bilge pump:
-One that has a separate manual switch.
-Moves a lot of water for the amps used.
-RELIABLE!
Here is a good article that supports the TWO BILGE PUMP rule at a minimum on any small vessel:
Those Essential Devices for Keeping Your Boat Off the Bottom
I know from working on the water for years, that many bilge pump problems can be traced to a number of factors, but specifically:
-OLD, with owners forgetting they have to personally check them a number of times during the season (preferable before EVERY trip)
-Crap caught up in the pump itself and not cleaned out
-Old wiring, and bad switches
-Broken float switches
-And the best, A PUMP MUCH TO SMALL TO HANDLE THE JOB OF CLEARING WATER QUICKLY!!!!!!!!!!!!!!!!!!!!!!!!!
Finally this one forum has a little discussion on this topic:
Bilge Pump Float Switch Arrangement
Most reliable bilge pump\float switch
GOOD LUCK!
EC NEWELL MAN<>