How is your ductwork currently configured. Usually, coming out of air handler, the main supply splits. One branch runs in the basement, supplying 1st floor registers, while other branch rises to supply 2nd floor.
What do you do:
1. Install motorized dampers on both branches
2. Install a zone control panel. Wire it to the dampers.
3. Wire existing 1st floor thermostat to zone control panel.
4. Install a thermostat on 2nd floor, run a wire to zone control.
5. Wire zone control into your furnace/AC.
When either thermostat calls for heating or cooling (in the unlikely event thermostats are making conflicting calls, "first come, first serve" method is used), zone control signals the HVAC equipment and opens dampers to all zones requesting that type of conditioning.
One place to check:
Should be a whole lot cheaper and easier than installing a whole new HVAC system in the attic.