Not an HVAC tech, but if you want the best performance it helps to run returns from each room that has a duct going to it. Lots of old houses have a duct or two going upstairs, however there is no air circulation so it still feels muggy and humid.
My house was made in the 1850's and the previous owner had send and return ducts ran up through a corner of one house to the attic, then ducts run to each upstairs room.