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.