One at a time, or two 30 amp connections.
Andy suggested adding a second 30 system for a second AC when I was considering that addition. It would be easier than converting to 50 amps. You can use a 50 to 30 amp adapter to power one of the 30 amp systems since most (all?) sites with 50 amp connections also have 30 amp service.
I haven't added the additional AC and 30 amp system, but this time of year I get that funny gleam. In the evenings when the sun sets, the 15,000 Btu Carrier I put on (see http://www.airforums.com/forums/f427...ead-40795.html
) keeps it very comfortable.
In the worst conditions, lots of direct sun and high temps, I close off the bedroom and wear shorts and minimal shirts. Fans also help, but I really am gone most if the time during the day.