I guess I should've clarified myself. I had been speaking of the Atlas 4200 and not the LS motors. GM indeed did design a twin turbo Atlas 4200. It never made it into production, instead they opted for the 6.0 LS for the Trailblazer SS.
http://www.motortrend.com/future/concept_cars/112_0203_chevrolet_trailblazer_turbo/viewall.html