Sure, extremely simple with common alternating relays and built in with the float assemblies. What sort of questions???
http://www.tesensors.com/products/nema-square-d-pressure-switches (Square D 9038 Series)
Here's just one mechanical example: A long rod from the float runs through the actuator lever with a collar above and below. The bottom collar is used to set the lead pump start height. The pump drains the sump to the height set by the upper collar. The switch will start the 2nd (new lead) pump the next time the water rises. Should the lead pump not be able to keep up, perhaps a check valve on the non-running pump has failed, the lever will rise to a second stage where both sets of contacts close. This causes both pumps to run until the upper collar switches them off. They don't alternate when this ocurrs.
For relay methods, duplexers and triplexers, are other names for the alternating devices. Here's one common brand.
http://www.marshbellofram.com/diversified-electronics/types/pump-level-controls/Hope this helped!
Joe
