Dual A/C's, but only 1 duct line

Depending how you are parked in relation to the sun, particularly the west sun, I would not try to block any duct. If it is hot enough for a need for air conditioning I’d just run both. If it is like ours when the thermostat is satisfied it will shut down the appropriate unit. If really hot our rear unit could cool the bed room way down and the front one, running continuously could not quite keep up. We turned the bed room way down it and used a 9inch fan on the floor of the bed room and circulated air from the back to living area. Any way I’d run both and set the thermostat at the desired temp and leave it alone.