We replaced the thermostats with thermostats that max out at 72 degrees.  

I haven't yet targeted specific commuting professionals, but heard a podcast where the investor advertised offering tech packages to attract young professionals: usb outlets, smart doorbell, lighting and thermostat.

The upper tenant has complained to me multiple times about it being too hot in his unit because the bottom tenant does not actively manage the thermostat.

@Svend W. the most practical solution to be 99.9% sure is to have a small electric heater on a thermostat.

Boilers don't run continuously--when the thermostat calls for heat, the boiler fires up. 

WiFi thermostat.

So, we know that we want to at least add some zoning control so that both tenants have some level of control over the thermostat.

Hire a good house keeper and between VRBO and the house keeper 85% of your running a short term rental are taken care of.You could hire out the lawn care too if your brother doesn't want to deal with this.I use wifi thermostats to save on electricity when no one is at property and a lock box for keys.I see you say you don't have a pool.

Rently can be nice if you set it up to include the smart lock and smart thermostat.

If you go this route hide a lockbox for emergencies.I also have wifi thermostats so I can save on electricity wen no one is there.I also have a high end outdoor only camera system that sends me alerts on motion so that I can keep an eye on who is coming and going.Once you are set up it is pretty easy to self manage.I have HA/VRBO and Airbnb and the calendars synchronize to stay updated.