Yep, the units do have two drives in them, but as they're really made to run as one unit, you're essentially getting one drive.
Walter Biscardi, Jr.
Owner/Creative Genius, Biscardi Creative Media
http://www.biscardicreative.com
G4 DP 1.25 w/FCP 4.1.1 & CineWave 4.5
G5 DP 2.0 w/FCP 4.5 & Aurora PipePro