Wiring my cabin for state inspection, I realized AC neutral will be bonded at Earth potential. Design simplification will be to connect the AC neutral wires, and switch only the 120VAC power (black,) lines. The switch component can be an SPDT switch, the normally closed side connected to my septic pump. This will simplify the design, and installation.

That AC neutral is bonded to Earth for safety anyway, precludes thinking there may be a "safety reason" to switch BOTH hot and neutral lines as in pump switches...

I guess it is useful to look at connecting the deep well pressure switch directly to the SPDT relay. When triggered ON, the well pump operates. Otherwise, power is connected through the normally closed connection to the other (septic pump and sensor,) pump. This design uses no logic, or one-shot timing pulse and will mimic functionality of a grid-tied system that allows both pumps to operate simultaneously. One will hardly notice any difference.

I'll try this first, as it is simple, and it may be in existence in some form. I'll draw the schematic in EAGLE and look at doing the board. An ample 12V wall wart supply could be used to provide power for the SPDT relay which will also be wired directly to the pressure switch.

Right now, my new installation has each pump connected to its own 15-amp breaker in my main panel of six. Simplification, and use of my idea to wire the SPDT switch will allow use of a single breaker, and eliminate having to pay attention, keeping the water switched off to avoid startup conflicts.