I'm just thinking out loud... If you are only controlling one freezer, the second freezer will only come on when power is provided to the first. So, if it warms faster for any reason, but only comes on for as long as the first freezer is running, it may not cool down enough. Or maybe vise-verse is the case with it cooling faster than the first would. Thus it may not be as controllable as the first. It is worth a try.