So you would have it electrically disconnected from the other circuit then?
I would suggest a double pole, double throw relay (for each thermister).
Have the thermister connected to the common leads, the NC leads would go to the existing circuit and the NO leads to your circuit.
You would then need your own circuit for powering the relay and for powering the thermister.
I have to go shovel some snow (northeast blizzard of 2010), can draw a simple schematic later if you need one.