The t-stat that failed was his central heating/AC for the apt.
To answer you question though. To back up your t-stats...
-Plug UTH into t-stat A
-Plug T-stat A into T-stat B
-Place both probes on UTH
-Set T-stat A to 91deg, set T-stat B to like 96deg.
You can also use a dimmer to back-up your thermostat...
-Plug UTH to dimmer. Set dimmer so UTH maintains about 96deg.
-Plug dimmer into T-stat and set t-stat to 91deg.
Or, you can just buy the Double Ranco's which are already wired to back each other up...
Rob