How do I efficiently run 2 a/c units for a 2 story house

Now that it's starting to get hotter we are about to turn on the a/c, fans just aren't working anymore. I've never lived in a house that was 2 stories nor one that had 2 units, one for downstairs and one for upstairs. This seems to be a pretty common thing here in Houston. My question is... how can we get the most efficientcy out of the a/c without breaking the bank? DH suggested running only the floor we are on, for example during the day we mostly are just downstairs, so we would just turn on that one. But is this effecient or would it make the a/c unit run more often? I understand hot air rises, so wouldn't when we decide to turn on the a/c upstairs make it work harder and longer? Suggestions please! I feel so lost!