Both probes go on the heat tape. The heat tale plugs into the main thermostat... The main thermostat plugs into the failsafe thermostat (this is why it must be a on/off style thermostat) and the failsafe thermostat plugs into the wall.
Basically if your main thermostat fails the failsafe thermostat (which is set higher than the main thermostat) turns the main thermostat on and off to maintain safe temperatures.
Like so:
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