I first must say that I am not an HVAC engineer, however I have a 4000 square foot house 2000 0n the main floor and 2000 in the basement.
There is a Central unit for both floors located in the basement. When
I bought the house the top floor had a return air duct located
upstairs. When the the temp reached the hi 90s or even 100 the best i
could get was 88 to 85 i call the AC pros and they said that my units
was under sized and wanted 8k$ for a better AC unit.
I theorized that if the basement was naturally cooler that if I pulled
the return air directly out of the basement that the return air would
be pulled down the stairs and cool before it was processed in the AC
unit. This tuned out to be true I no longer have to run my basement
HVAC an the upstairs runs a cool 72F.