Generally the second unit is a non-ducted unit. The ducts cut into the ceiling are not large enough to handle the additinal air volume of a second unit. You need to be careful closing off vents as you may reduce the air volume through the unit and cause freeze up problems.
Have you got insulating covers for the vents? Also, cutting the foil backed bubble insulation to fit windows will help reduce the load as well. Your best friend is shade. Keep as much of the trailer as possible in the shade during the hottest part of the day.
A 32' is pretty much the limit for one A/C unit whenused in hot climates and direct sun.
If you are getting a 20 dF difference between the inlet air and the outlet air, the unit is working as designed.
As for cuttting a hole to mount a second unit, I noted the ducts are two small plus, the roof is not designed for that much weight in the one area and probably not braced.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot