Optimize HVAC for the SpaceMigrating one's STR's pellet stove + electric baseboard heating to a $5k 28k DIY mini-split + baseboard dropped my electric bill 50% in a New England STR, but that's with extensive automation of heating cycles, retrofitting of electric heaters w/smart thermostats, and understanding that splits / heat pumps do lose effectiveness in their ability to extract heat from very cold air.

He suggested tenants not to turn on the thermostat any more as it may cause fire and carbon monoxide etc. 

Again, anticipate immediate repairs being required.Lastly, while each of these systems is zoned and allows each tenant to have control of their own thermostat and heat setting, they are only connected to one gas meter.

Nest camera's overlooking the front door and Nest thermostats controlling the A/C.

Even if they do, just get a wifi connected thermostat and home control system and you are set.

You can also install programmable thermostats to try and control things.

Another issue is that the upstairs tenant has control of the thermostat, so if the lower level tenant wants heat, they need to call the upstairs tenant to turn it up or down.  

My maintenance crew visits in the winter to change the furnace filters and explain how to use the thermostat.

I installed smart thermostats in my rentals and they let me monitor humidity in the rental units remotely.

I have a NEST Thermostat, so I can turn down the temperature remotely and even when my long term renters go away, the thermostat knows they are gone and turns down the temperature automatically.