Is it really possible to design a single HVAC system that will cool a single story home equally in all rooms throughout the entire day? Wouldn't 2 systems work infinitely better to adjust to the changing conditions during the day (for example, one east side and one west side)? It seems to me even if the system is designed perfectly (which it never is) to provide worst case air flows to each room that you will either be under or over cooling parts of the house at certain times during the day.
I have owned 3 homes in Central Florida all 1 story, all 1400-2900 sq.ft., all 1 hvac unit, all had uneven cooling/heating in the rooms. It seems the builders here only go to 2 units for over 3000 sq.ft. or 2 story. I think the reason they go to 2 units over 3000 sq.ft (1 story) is because the requirement exceeds 5 tons, nothing to do with comfort.