Sorry the above post confuses me..... why would you have a float switch if the water level stays the same in both (both what?).
You want your switch wherever your evaporation becomes evident... for me it's the last chamber in my sump. The other switch is in the bottom of my 5 gallon bucket of limewater for topoff (which that switch really never gets used.. as it never goes empty thanks to me).
If you don't have a sump then you just want it at the top of the water line in your main tank and wherever you set it and trips the float becomes your new water line as when it drops it should fill till the float ... well floats.
Does that make sense? I hope thats what you were getting at.
The OP has a AP24 witch has two built in chambers for filter media, heaters, return pump, etc... I have the exact same tank and the water stays the same in both chambers becuase of the way it was built. So either chambers would work.