I would suggest separate systems for the two levels as both levels are unlikely to be occupied at the same time. One thermostat controlling a whole house with different levels of solar exposure is not a great solution for a luxury condo.

Zoned systems in residences tend to be problematic. Two separate systems work MUCH better. Often they can be more efficient with each being smaller. Very importantly if one chokes a residence still has a cool area available for the victims.

Keith Cress
kcress - http://www.flaminsystems.com

Two systems on each floor is best if in the budget. Zoned systems with only dampers don't seem to work too well. If you do one unit for both floors just make sure it has an ECM motor and specify the fan to run 24/7 to make the temperature more even on both levels. You'll want a return grille on both levels as well.

It all depends on the owner and what they are willing to pay. 2 zones obviously will provide better comfort and likely some energy savings, but will cost more upfront and likely requires some more space. so what we here think is not relevant - the owner has to decide if the benefits are worth more to them than the cost.

With more modern variable speed fan and multi-stage burner and AC it can be done with a single unit. Carrier et al have ready solutions for such zoning and the appropriate furnace/AC AHU. If you just want to use a cheap single-stage type, 2 separate AHU may be better.