I think it is twin sequential turbo's meaning a smaller one before the bigger one to help the bigger one spool faster, less turbo lag that way, if any at all.
I looked up this engine and it sounds really awesome yes the engine is dual turbo but like Don said there is a smaller per turbo that feeds in to the larger turbo and the engine makes god awful power at low RPMs 650 lbs at 2000rpms thats just awesome
I have placed an order for a 2010 f-450 instead of waiting for a 2011 because I didn't want a first year new engine with all of the anti-pollution junk they have come up with to satisfy the idiots with the epa.
I have a job 3 2008 6.4 and would not trade it for anything in the world. Only complaint is poor fuel mileage. Not Ford or International's fault though, just playing the hand they were dealt.