motoguy128 IF the one guy did a load calc and it supported 40k BTU"s then that's probably all it needs. 1600sqft on 2 floors isn't very large. Since it's two floors, it's not going to need as much heat as a single story 1600sqft home since it has a single roof 1/2 as big. half the property is underground. I'd bet the load cal called for something like 35-40k BTU's.
For splitting the system. It might be possible to install a simple zone control system and just not use and dampers. That way at least each area can "Call" for heat independently... then if it's too warm most of the time the tenants can partially close a couple dampers. IF you're paying for the heat, you don't want them opening windows.
Another option is getting a thermostat that has the option for a remote indoor sensor. It can at least average the two temperatures so you will at least split the difference.