When do I tell tenant they dont control the thermostat

[Frank Mwaisaka]

I am a new landlord with one up and down duplex. The building has separate electric meters, one gas and water meters. For that reason I am responsible for water and gas bills and tenants pay their own electric. Because of having only one gas meter, there is single HVAC hence singe thermostat as well. The downstairs unit already has a tenant and I am getting ready to rent the upstairs unit. When is the right time to notify my prospective upstairs tenants that they do not have control of the thermostat, it will be controlled by the tenant who rented downstairs?