My brother’s house does have a good unit

My brother and my sister-in-law recently moved to a new house that is only about ten minutes from a major port, beach, marina, and fishing area. They have a boat and a swimming pool. In fact, there isn’t much they don’t have. If my sister-in-law wants it, my brother gets it for her. He has always been very generous, and he still is. I admire that about him greatly. Many people choose to downsize when they are preparing to retire, but they bought a bigger house! Their house is so big that they have two HVAC units. They have one air conditioning unit that is for the upstairs and a second HVAC unit that is the heating and cooling for the downstairs living area. I thought it was very modern, but it turns out that having two HVAC units is not really ideal anymore. Nowadays, any good HVAC service guy would tell the homeowners that the best heating and cooling system would be one HVAC with zone control rather than two separate HVAC units. I get that. The benefits of only slightly cooling the upstairs while cooling more aggressively downstairs makes a lot of sense. The same holds true in the winter. Why have the same heating and cooling for both areas of the house when no one is ever upstairs? They really only adjust the heating and cooling upstairs when they have guests who are staying up there because their bedroom is downstairs. Whether they have separate units or HVAC zone control, I think it is smart.