The sensor could be as simple as float switch in the sump, if main pump fails sump level rises and triggers float which activates back up. Obliviously need a little coding to ensure it doesn't turn back off again once sump level drops or doesn't turn on during maintenance shut downs but that should be pretty basic. Alternatively profilux has a flow sensor available, perhaps it can be adapted to Apex or you could purchase another type of flow sensor that could be adapted.
If it were me I'd just run both pumps together, size the pumps or restrict the flow so each pump puts about 50% of the desired flow to the tank, if one fails you're left with 50% flow which obviously won't effect much. Then there's no controller or sensors to rely on and you achieve the same benefit with less reliability concerns.
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