You didn’t provide enough info to give any good answer. Sounds like there is only one thermostat and it’s on the first floor, which is open to the second floor and shares the unit. Where is the air supplied to? Where is the air returned from? Does the unit coil as well as heat?
Is the supply air from the floor or ceiling? Delivering air as low as possible will help during heating, but you may get the opposite problem when in cooling mode
You may not have the highest level of your space sealed, which in the winter creates a chimney effect drawing in cold air through the lower levels as the hot air leaks out the top
Ceiling fans run in reverse can also help mix the air better, but may not push with enough velocity to get the warm air all the way down to the first floor
You could put in an independent inline fan that comes on at the winter and draws in air from your high/hot point, and delivers it back down to the lower levels
Is your space intentionally open between the two floors? If easy to do you can seal it up and make it harder for the hot air to migrate up.