There are lots of great replies here, but without specific numbers it's hard to say for sure whether you'd be happy with one or two a/c units.
My wife is happy as a clam even with no a/c at all. She grew up in a very hot area and her parents didn't have a/c in their house. As a result, she'd probably tell you that one a/c (or maybe even no a/c) would be just fine. As long as it's below about 85ºF, she's happy and prefers silence rather than a noisy a/c.
I, on the other hand, grew up in the north and if it's more than 75º I feel hot. I'm happiest at 72 or below. If it were me, I'd probably get the 2nd a/c unit on the off chance I visited the southwest in late spring / summer.
Bottom line, one person's "it's fine without a/c" may be somebody else's "it's sweltering hot without a/c!", even though it's the same temperature for both people.