Your problem is that you do not really have two floors, you have one big open area about 20' high. Heat rises and cool air descends. This creates a logistic problem over a 12 month period. In the summer the AC air works best if it is injected at the lowest level and then rises as it warms up. In the winter, the ideal is to "suck" the cooler air out of the lower level send the warm air in high so it will disperse its heat as it descends, is the exact opposite of the AC needs. But, if the units run constantly without satisfying the thermostat setting, they are too small for the house's heat loss. Otherwise at least one would turn off if any area of the house were getting cool.