My wife and I purchased a 2018 M325 in July of this year. I'm curious about a second A/C unit. I'm assuming you added it due to lack of air flow in the bedroom. 1) I purchased a small lighted scope camera that attaches to my phone. I removed the vent covers and inserted the camera into the ducts in the ceiling. I found in one location a fairly large blockage. There are wires that run from the floor into the ceiling through a large hole. When they cut the hole for the wires into the duct, a large piece of the foam insulation got pushed upward and was acting a diverter. Unfortunately, it funnels the air downward into this hole in the wall. I'm not sure that once I get this resolved it will resolve the issue but worth a try.
2) In case I need to install a second A/C, did you put in a ducted or non-ducted unit? My TT is prewired for a second unit.