@BrianDipert Do you have a picture of the wireing of the old switch?

@S2005 If you just change the one switch in box1 you would not need to run the romex

@S2005 Mark the existing white that goes from box1 to box2 with black tape(so you know it is a traveler) Run a white neutral from the other whites in box1 to box2 to be used for box2 HS210...

@Steve_______ Because it is low voltage, it should be OK as long as you do not put it in the same box with house wiring.

@S2005 In Box 2 connect black to common/load, red and white are travelers. You will need to run a neutral from box1 to box2. In box 1 connect black from Hot In to common/load, red and white are...

@Azazel Without a neutral you are out of luck