Had this conversation before, but the engine itself is a fixed volume. The flow at any given boost value is the same. The only way to change anything is in terms of mass, and if you lower the air temperature, then flow you do have would be more dense Then you would get more flow through the turbo itself, but the engine is still only a fixed air pump as far as volume goes. The main advantage of the twin for flow though, is its not as restrictive as running both banks into one manifold. So the twins are less restrictive and makes the truck more efficient.
Two GMx turbos, each at ~10cm^2 turbine housings, is about 20cm^2 worth of turbo. The 6.5 cannot drive much more than that. If anything, a couple 9cm^2 HX35s might work alright if quick spool is a concern, otherwise a couple of the newer 12cm^2 HX35s or similar sized turbo might be better for efficiency. I think Heath's turbos were small.
