I recently purchased a new home with two zone ac one covers the upper levels and one covers the lower level. I have been having an issue balancing the lower level unit airflow. One room is very cold with very high flow and other rooms on the lower level are warmer with much lower airflow.

When I checked in the attic, the higher flow room ducts are much closer to the output from the air handler than the ducts I am having low flow which sit at the end of the header.

I am not a professional so I would appreciate guidance on fixing this.

Was also wondering about operating the system with respect to running both units or only one at a time. It seems in the heat of the day, I need to run both to keep the house cool. Note both air handlers and returns are in the center hall and reside in the attic.