Well that may depend upon the duct system, I would turn them both on full heat as high as they go, then about ten minutes later use a thermometer and see if there is a major temperature differnece between upstairs and down stairs. If you are living mainly down stairs, that area is the one critical for temperature settings, because heat will rise and the up stairs will get somewhat warmed from the lower t-stat operation any way. Then all you have to do is determine your average temperature diference and set the stat to compensate.