We have a new 2 story house (2 units) without cathedral ceilings and have no trouble keeping it at that you set it too but I don't let it run all day when no one is home. I turn it up to 77-78 and back to 75 when we get home. It takes an hour or so to cool back down. At the house we sold last year we could never keep it cool in the summer. It was a smaller one story but had lots of cathedrals. With the kind of heat is was always at least 78-80 in there and nothing was ever found "wrong" with the unit. It did that from day one. Maybe it was undersized but I am not sure. I am sure liking having 2 units but maintenance/repairs will cost more obviously.
The chances of them both going out at the same time are slim. At least part of the house will be tolerable while you wait on the repairman. 8)
We have a ranch, w/ cathedral ceilings, so maybe thats contributing to the problem. Normally it does fine - it's just been with these extremely high temps that we've been noticing a problem.
I love you Arctic Air!! It was just some switch, a 122.00 job. We have probably spent 7 times that on electricity over the past year. If I were an "I told you so..." person, I would have told hubby "I told you so" lol