I went through the same dilemma as you. My 13.5K unit just wouldn't keep up on hot days. I too had the prep in place for a second ducted unit. In fact, the manufacturer included wiring for the thermostat by hiding it behind the bedroom light switch. I had mine installed, but it's relatively easy to install them. The big issue you might face is how to get it up on the roof. Those rascals are heavy.
With the old single unit, I'd get up to 85-90 on a hot day here in AZ. With both units, I can easily keep the rig at 75 or lower.
Jim
