We have two A/Cs and there are three reasons I'm REALLY glad we do: 1) for now, our health care providers are in Georgia. It gets extremely hot and humid there in the warm months and the two A/Cs have been necessary to keep the inside temps comfortable. There have been some "two A/C days" in the desert southwest, as well; 2) in the evenings when it cools down, we turn off the living room A/C and just run the one in the bedroom. They push the same ducting system and this really cuts down on the noise in the LR to watch TV, talk, etc.; and 3) the comfort of knowing that, if one A/C breaks down, we won't be completely without while we get it fixed.
Rob