I know the alarm is easy to do but two heaters completely independently? Not sure what you mean. If you want one never to be on except if the other fails then the easiest would be to just set the actual heater thermostat a little lower than the tank temp. That way you could probably also benefit from another alarm for low temp.
You could also use the bottom heater. You actually by default have two temperature set points, when temperature fails below the first the heater activates and if the heater cannot keep up and the temperature stills falls and reached the second set point the bottom heater activates. The set points are determined from the hysteresis.
You can also create virtual probes, in your case temperature probes. This will allow for endless possibilities for controlling multiple heaters and coolers at different temperatures. However I've never played around with it so can't say much more.
The best place for info on this stuff is the NA profilux forum.
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