if your going to be running two sets of flexwatt at two different temps you'll need two probes so the herpstat II would be a good choice.i have two herpstats II's and am pleased with there performance.they have a hi and low temp alarm so a backup would seem like overkill,also the unit is designed to shut down in case of failure.

have you considered a oil filled electric heater in your room to raise temps 5-10 degrees? then cool side flexwatt would not be necessary and you could use the extra wattage for other racks/enclosures,just a thought.have fun!