Cheap but Reliable Thermostat?

[Chkadii]

This is slightly off topic, but... How do you set up a backup thermostat?
I have a herpstat 2 on my rack, and the heat tape is wired to a lamp cord, which plugs directly into the T-stat. Where does the second T-stat come in?

Both probes go into the same place against the heat tape. The heat tape gets plugged into the first thermostat, which is set for your ideal temp (let's say 90F). That thermostat gets plugged into the backup thermostat instead of the wall outlet, which should be set to on/off mode and have a temperature a couple of degrees above the first thermostat (93F).

Since the probe won't read 93F while the first thermostat is working, the backup will give the first thermostat full power. The first thermostat will keep the temperature at its ideal setting. Should the first thermostat break in the "on" position the backup won't let it have enough power to heat the tape past 93F.