I have a great room (kitchen, family room combined with cathedral ceilings, two vents over the kitchen (from above, handler is in attic), and two over family room section, all evenly spaced.
I thought they were all run off the 4 ton unit, which covers this room, dining, living and master at far end, but I noticed the family room ones weren't blowing much at all, and upon checking I found that these two were connected to the 2.5 ton unit that covers the 3 bedrooms and bathroom at that end of the house.
I think this might be a later retrofit from the original spec on this 40-year-old house because it is connected into the main run by an arched flex pipe about 12-15 feet long like a giant slinky over and through rafters, which seems like a long way to go to the section of box duct from which the two left registers are supplied. As a result, temperature is 6-7 degrees warmer than the vent before the arch.the pressure is almost non-existent, and probably affecting volume to the bedrooms as well.
My question is is this likely a retrofit? Does it make sense to have 4 vents into the one room, supplied from 2 different ACs (2 and 2)? It seems you'd only be effectively cooling that space when both ACs were active, and otherwise, only half would be being cooled leading to an overall warmer area.
You are correct, you shouldn't really have 2 different systems cooling (heating) one room. I suspect originally only one of the two systems was supplying conditioned air to that room but it was insufficient. A better solution would have been to add and extra supply and/or return from the original system.