Air condtioners can't work too hard. They are either running or not. For best efficiency, they should run almost continiously in hot weather. Whatever you do, don't do it so that one or both continually switch on and off. That is known as short cycling, and it will cost you more money in energy and shortened system life than anything else.

Short cycling indicates too much system for your needs, and it usually results in poor humdity removal.

If you can, set things so that one unit is on almost all the time and the other one never runs.

Thanks Frog Hopper, this makes sense. Maybe I will get an AC specialist to check our system and make sure all is well and running as it should--our electric bills have been terribly high lately.

Having a checkup would be a great idea, especially if you haven't had one done in a while. Perhaps they will find something that is a plausible cause of the high electric bills--incorrect refrigerant charge, dirty coils, etc... Call a reputable company and ask for a tune-up. Well-maintained systems tend to have a longer lifespan.

Thanks Ryan!