Just for discussion purposes if you wanted a reliable, cost effective method of staging several units with a single controller check out honeywells line of T775 controllers I have used them in several applications and been very happy. Mount the controller out of the way somewhere and just install a remote space sensor. They make one model that will control up to 4 stages off of a single controller.
a fistfull of relays, boy that is scary:rolleyes:
some tech's, are not working today
there are plenty of ways, with a few "or less" relays
but this would need to be discussed in the appropriate forum