First, they can set to be either normally open, or normally closed. In fact, you could put two circuits on one at the same time, and when an appropriate pressure was reached, the open circuit would close, and the other the reverse. They are pretty flexible. And pretty accurate.
Here's the problem...
Let's say it knows that the pressure threshold has not been exceeded (water column not high enough) and it closed a circuit, turning on a pump to put water in the tank. The pump will remain on until the appropriate height is reached, then the switch opens the circuit, and the pump turns off.
On the next day, you drain a bit of water out of the tank, and assume the switch will close the circuit again and start the pump to replenish water again. Nope. Once that that pressure sensor opened the circuit (the day before) it will not close it again until the water drops about 10"!
So yeah, I've figured out a kludgey plumbing mechanism to fool it into thinking that has happened. But it includes solenoids, drains, and just way too much wackiness. There has to be a simpler way.
I've thought about using both of them, as you suggest, one as a high, the other as a low... but it doesn't really work, I don't think. No matter what you rig one to do, turn on or off, at a given pressure, you still have to lower the pressure about 10", where it switches to the opposite state (off to on, or on to off) to get it to do that trick again.
It could be the solution to my problem is a refund.
Steve - 33g reef and a 180g planted in need of a re-scape.