Hey there, I personally believe running a backup thermostat is a great idea and provides extra safety and piece of mind. Something alot of people worry about is their thermostat failing and cranking up the temps and killing their animals (as temps being too hot are much more deadly than too low temps).
In order to set up a primary and back up thermostat system you need a proportional thermostat and a on/off thermostat.
For my back up setup I use a Helix proportional thermostat with a Ranco on/off back up. Basically all I do is plug the on/off thermo in the wall and set it about 5-8 degrees higher than my desired warm side temp (whatever you set it to will be the cut off temp)and then plug my proportional into the on/off thermo and set the proportional to my desired warm side temp and plug your heating element into you proportional thermostat. Now tape the 2 probes together and place them in the proper spot. Now if you proportional thermostat breaks and starts cranking up the heat your on/off will turn the power off to it once the temperature reaching your cutoff temp.
You can also check out more info about thermostat backups here http://www.reptilebasics.com/thermostat-faq.html
And one more quick note. If you set something like this up make sure you do not use a UPS or generator as a power source as it will mess up the proportional thermostat.
Hope that helps
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