[HayZee518]

ok, there's one way to do it. with an electrician's snake, see if you can drop the snake along side the stack in the attic all the way to the basement. you might need to shake it twist it and let it fall do anything to get around the cutout in the plate(s). another area you may be able to "snake" down is next to a chimney. most times the carpentry or framing leaves around 2 inches all the way around the chimney. once you get the snake to the basement, then all you need to do is calculate where or rather which wall you want the thermostat partition in. then figure it out in the basement and drill up in the wall partition with an eighteen in auger. make a hole in the wall and use a piece of string with a small weight on it and drop it into the wall space all the way to the bottom. then use a bent coathanger to see if you can snag the string and pull it through the hole in the basement. now you got a complete run to the attic. hook a piece of 18-2 or 18-3 and just pull up to the attic and up into the wall space. wiring is just disconnect the upstairs stat and reconnect the lower stat wires.