Have 85,000 BTU forced air furnace which supplies two separate 500 sq.ft offices.
Working hours are different in each space with only one thermostat in space one. Can I add therm. In space two supply and return air are not a problem. Adding therm in space two will alleviate the problem of tenant in space two from unlocking and entering space one to turn on furnace.

Not without adding dampers in the duct work and installing a zone control box. You will most likely need a sheetmetal tech, and an electrician to make this work properly. But yes it can be done(depending on current duct work layout). May have to reduct furnace. Can you give a detailed description of the ductwork layout?