Is the other space a full time occupancy?
If it is not and you are looking for occasional use or if the garage wont be used daily the cheep fix is to wire both stats to the unit and leave the one you are not using "off"then run your duct to the new space teeing into the existing ductwork. you will need 2 manual dampers one on existing and one on apartment.

If it is the better solution would be to have 2 motorized dampers that are wired to the stats. If one calls it opens the NC damper and runs the unit. If 2 call both NC dampers open and the unit runs. This way is better and will allow you to have the shop at 60 and apartment at 70 in the winter.