If you are looking to stack a pair of counter-rotating fans, looking for increased pressure performance at lower noise levels, I think you are going to be disappointed in the noise department.
I tried this (just casually) a year ago or so (so just one data point). The fans interfered with each other acoustically and were considerably louder than just the two fans side-by-side (experiment done in clear air, no resistance - what axial fans are good at). Note that I was aiming to get fan noise under 18dB or so and was thus undervolting by a lot - I'm sure things are different as you move the fans more into the performance envelope they were designed for
The SD PDF mentions that they have gotten the noise down to the single fan plus 3dB (a 3dB increase is what you would expect if you add two identical noise sources together). IMHO this is quite an acomplishment. Their diagrams are fairly schematic, leaving me wondering what they did to get this result (different fan blades? flow straighteners - like in a multi stage turbine?) The spacing between fans is probably critical - and may vary based on something dynalic like airspeed or blade speed - so even the ones SD designs might be poor candidates for undervolting.
It would be nice if they spent some development money on radial fans that were drop-in replacements for axial fans. Unfortunately the easiest way to do this leaves you with an impeller, not an "expeller" so suck not blow, which is inappropriate for a lot of PC cooling (except for radiators, but we're in a distinct minority with those).
Oh - I've also done some experimenting with under-120mm blowers and motorized impellers. Yes they certainly generate more pressure tha equivalenmd sized axial fans - but they generate more noise, even when undervolted to the point of equivalent airflow in clear air (still superior for overcoming resistance like rads, but not quieter). I've come to the same conclusion as cathar - use a radiator with large frontal area and little thickness - and couple that with quiet axial fans.
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