Its the other way:
Flexwatt -> herpstat -> hydrofarm -> wall
This works best with single channel thermostats like the herpstat 1 or intro but it will work as well on a multichanel thermostat as well (like the herpstat 2 or 4) but whatever is plugged into the other channel(s) will be held hostage by the hydrofarm in the case of a failure.
This works because the hydrofarm is an on/off style thermostat. It allows power to get to the herpstat as long as the set temperature on the hydrofarm isn't breached. If you plugged the hydrofarm into the herpstat as soon as the herpstat reduced power to that channel the hydrofarm will go dead (or worse start acting erratically)