If you are borderline with a single Air/con, a second unit, even if the smaller 13.5btu should easily keep up.
One thing, with the single unit, can you shut off the vents to the main living area at night? With our old unit, by opening and closing vents, we could pump the cold air to the area we were in (bedroom at night and living room during the day). We would do it on very hot days and that let the single unit keep us comfortable. Unfortunately, out new rig doesn't have vents that close, so we have to figure out what to do with it.