2 Units with 1 thermostat?

[Tarik Larue]

Greeting BP family,

I recently bought and rehabbed a 2 unit townhouse with separate electric and gas meters.  However there's only 1 water heater and 1 furnace to heat the water and radiators (its an older rental). The thermostat is located in one unit to control the heat for the entire house. Not sure what my options are. Do I buy and install a second furnace which sounds costly? Perhaps I can install a thermostat that only I can control? Then I will just pay the gas bills and split it among the two tenants. Experienced landlords and property owners please advise me on my options. Thanks