You've got it backwards. With a smaller feeding into a larger, you can achieve much greater throttle response, and directly more low RPM torque. The smaller turbo will spool much faster, and will feed the larger turbo, which will create a fast spooling, high volume boost system. This is the best route to go in my opinion.
You may have to do it that way for throttle response. If you have a two stage centrifugal compressor the first stage is the largest for a higher flow and the second stage is smaller for higher compression, both stages turning the same RPM. That is what I am use to seeing and it don't sound right the other way. But I guess it works. Do they both turn the same RPM?
I must say that first ford concept, the Reflex is pretty good looking IMO.
I myself am not looking into a diesel yet, but just reading about the introduction of a DT diesel is pretty wild considering only 20yrs ago most diesel trucks were new to the market and non turbo, now we've got TWO turbos mounted onto a V6 and its a factory job. IMO thats a bit of a wow factor.
In series turbocharging, the smaller is dubbed the high pressure turbo, and the larger is the low pressure turbo. On the compressor side, the larger feeds the smaller. On the turbine (exhaust) side, the smaller feeds the larger. The turbo is a volumetric device - requires larger wheels for larger volumes (lower pressure) and smaller wheels for the same flow at higher pressure.