Here is a couple compressor maps
Here is a gt55 map, notice how even with a cast wheel its capable of a 5.5 pr (not that thats ideal)
http://www.turbobygarrett.com/turbobygarrett/turbocharger#
Here is a gt4294 with a 71mm Wheel, to compare to what nick is running, see its only good to about 4.2?
http://www.turbobygarrett.com/turbobygarrett/turbocharger#
And just because it runs two atmos, and two high pressures doesnt mean they are splitting the load, they are both taking in the same atmospheric pressure and compressing them to the same level, at both stages. Think of it like 4 four cylinder engines independant of each other.
The advantage to this set up his huge flow, but 4 lighter wheels/shafts compared to 2 heavier wheels, which will help spool up, and possibly make it spool more mass air flow than traditional compounds