[javadog]

You might also just close the registers in the downstairs area so that all of the cool air is directed upstairs. The downstairs half should remain cool enough. Adjust the exisiting thermostat accordingly and you're done. I have a two story house in Tulsa with seperate systems for each floor and, while it doesn't get quite as hot here as it does in Houston, we never run both systems in the summer or winter. In the summer we run the upstairs unit and in winter we run the downstairs unit. Works fine.

You can also adjust the registers on both floors to direct whatever percentage of air you want to either floor, or to specific rooms. This assumes you have adjustable dampers in each register...a little fiddling and you should be able to get an even temperature distribution.

JR