You can do it, Furnace control is normally two blue wires, You need a bi-metal or electronic T-Stat (I suggest the bi-metal type) NOT a mercury bulb type.
You will also need a "Front/rear" switch, a SPDT switch will do it,
Disconnect ONE of the wires from the existing T-Stat, run this one to the switch, Run a short jumper from the switch to the T-Stat
Run much longer wires from the other T-Stat furnace terminal and the other switch terminal to the bedroom T-Stat, Flip the switch for Day/night modes.
NOTE: This assumes the T-Stat ONLY controls the furnace, if it also controls the Air Conditioner you will have to find the needed wires. That can take a bit of searching.
