A programmable thermostat, receptacles that have USB ports; there are lots of things that improve your property, don't cost a lot of cash & are kind of nice to have. 

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.

Before making a no heat call please check that the batteries in your thermostat are properly installed and have a charge. 

For small spaces, you can't get a small enough (in terms of BTU) multi-zone unit and what happens is one unit receives refrigerant even when not running if the other is running, which due to the cold of the evap coil causes it to condensate and ultimately causes constant problems with drainage.Also, look for a mini-split where you can connect an external thermostat and configure the head unit to shut off when the set point is reached.

@Abigail NittleMy only addition to what’s been said is the huge advantage of a remote location to add “smart” features like thermostat and WiFi door locks that allow you to manage the space at a distance.

I can personally attest to that as I've replaced 2 HVAC systems and an Expensive Thermostat in another in recent months proving you need a CapEx fund or reserves. 

Then you have a blower in that room with its own thermostat.