It has one furnace and one thermostat in the down unit that controls the temperature for the entire house-both units.

You might want to put a timer on the thermostat that turns it up and down depending on time of day or charge them over and above a certain amount of usage. 

That way, you can just give 30 days notice to end a tenancy, as well as 30 days notice to change any rules, etc.The things that usually wreck roommate situations is unwanted guests, not cleaning up after themselves, messing with thermostats, stealing food.Some of those things can be pretty easily dealt with - like including a weekly cleaning service in the rent, putting little apartment fridges in the rooms.But, inevitably, there will be a roommate who you will want to get rid of, so having month to month agreements is the easiest way to get rid of them.  

If I plan on doing anything that would inconvenience them (paint the porch, replace sidewalk) I let them know 'way in advance.Twice a year, I change all the smoke/CO alarm batteries & double-check the thermostat batteries.

Ended up they had 2 bad dryers, one with a dead thermostat one with a dead heating element. 

Maybe a smart thermostat or something.Do not let tenants dictate your upgrades - they sound like a PIA, honestly.

Unless you get the utilities separated along with separate heating for each unit, you'll never find a 'fair*' way to distribute costs IF the tenants themselves have access to the thermostat, a/c, hot water etc.

So of course if the tenant set the thermostat to an unachievable temperature it may cause the unit to freeze up.

The thermostat is on the 2nd floor and is set to 72.

Then you meet them at the property, hand them the key and give expectations such as when and how to replace air filters, properly use the thermostat including making sure the fan is on 'auto' and never 'on', where the trash/recycling cans are kept and when to put them at the curb, etc.