Interesting setup. You'll occasionally see one on the used market around here.
What you get is the standard crew cab bed (5 ft) and the standard regular cab on a crew/xcab chassis with the piece with side doors stuck in the middle. The middle piece is open all the way across to both sides. Every one I've seen also included the work cap on it. If it suits your configuration needs, its especially cool.
For me, I would have preferred that GM had made a reg cab with a longer (7 ft) bed but that configuration was never available in the first run of Colly/Canyon. FWIW, I never saw this configuration in GMC form.