I added a second A/C to my 5th. It came with a single 13.5K and I added a second 13.5K in the bedroom. As you know, it's pre-wired for both the electric to the A/C as well as the thermostat.
The unit installs into the existing duct work and works independently of the main A/C. We use it for multiple purposes. Since my camper doesn't have the new quiet a/c, I will use the bedroom A/C during the day when people are in the living room to considerably cut down on the noise. We'll use both when it's really warm outside, and the most noticeable thing is a significant increase in the volume of air coming out of the registers.
Mine is tied into the pre-wiring, and we require 50A service to operate both at the same time, otherwise we're limited to 1 because of the draw on power.
If I had to do it all over again, I would without hesitation.