There are a few ways that you could work around this, depending on the setup.
If there is no crawl space, then the Crawl Space control set can be used to as a very close approximation for the floor over the garage. Be sure to specify it as an Unconditioned Crawl and if you want to get even closer, you can open the More popup and set the % Tempered by Indoor control to 35% or whatever setting you feel is best for the garage.
If that is not an option, but the two floors are identically insulated and will be improved in the same way, then you can lump them together in one area and adjust the % Temper control to reflect a weighted average of the two spaces (e.g. % Temper = 35*GarageArea/TotalFrameFloorArea).
If neither of these solutions will work for you, then please give me a little more detail about the condition of the floor and your planned improvements.