[Chris Jones]

Drew, the same float switch from grainger that I used to turn on my top off pump when the level is low can be inverted to turn off something else for the same reason. so in one orientation it is normally open in the floating position, for the top off, and if you invert it it will become normally closed in the floating position as for your return pump.
When the water goes down your top off pump will turn on, and the inverted switch will turn off your return pump. you adjust the depths at which these occur.
Good luck