You are correct- you can't do it. The board has just one clock for analog input.
You can sample all of the channels at 2000 samples/second, and either throw away the extra data (easy) or average the extra data down by a factor of four (harder, but better).
John Weeks
WaveMetrics, Inc.
Phone (503) 620-3001
Fax (503) 620-6754
www.wavemetrics.com