I'd just forget about the dual turbo idea for a S/6,those chinese copy-cat turbos on ebay are realy hit or miss on the quality.The money you'd spend on cheap duals could buy one brand name turbo of known quality.My GM turbo starts to produce boost at 2,500 RPMs.I lucked out and found a wrecked 95 truck with a like new turbo on it at the junkyard.A single turbo with an internal wastegate will be "less difficult"to fab parts for,and tune.Just remember,you'll be dealing with plenty of problems trying to get the carb tuned.