Zoning can be done. It certainly is not cheap and is and always has been complicated. Adding motorized dampers and such is a complexity that most residential installers know nothing about.
If you want a real challenge ask him about adding in a fresh air supply.
As others have said 2 sets of duct work, 2 compressors, 2 to-stats and 2 air handlers is the way it is done for most homes. I live in Phoenix and have never seen a dampered a/c system in a home, commercial yes.
Since heat rises the upstairs is always warmer in the summer. So that unit will run more. I once saw a home done with a 5 ton down stairs and a 2 ton up stairs for the master bedroom. There were no doors upstairs just division by halls and the stairs. In the summer time the upstairs unit ran 24-7 and the down stairs cycled not exactly what the owner had wanted.
I suggest that you start contacting more contractors. Look at some 2 story new builds in your area for some ideas.