The descriptions "short" and "long" fin stack are a bit confusing to me. I trust you mean a "thick" stack like the Thermalright HR02 vs a "thin" stack like the Prolima Armageddon:
Thermalright HR02 Macho
No theories regarding airflow & heatsinks; I can only tell you what I've observed over the years -- ie, empirical experience:
With a large, thick fin stack that has tight spacing between fins, there is very little airflow on the opposite of the stack even with a powerful, high speed fan. But you can feel a lot of airflow coming through from the sides of the stack (ie, perpendicular to the fan), as well as around the periphery of the fan where it meets the fin stack. This does mean lots of black pressure. It also means that airflow across the fins drops dramatically the farther you go from the fan. Hence, these types of heatsinks can work very well with a dual push-pull fan setup, especially with higher pressure/speed fans, which increases pressure so airflow reaches all of the large cooling surface area of the tightly arrayed fins.
Take a big, thick fin stack with widely spaced fins like the HR02 or Ninja, and there is much more airflow that comes through on the side of the fin stack opposite a single fan, and much less airflow around the perimeter of the fan/stack interface -- ie, less back pressure, more air flowing through the entire surface of all the fins. These designs benefit much less from a dual-fan push-pull setup.
This applies to large, thin
fin stacks like the Armageddon. Ditto the unique 5 fins stack array of the Mugen (I don't recall any other HS using this design.) The high impedance that a single
fin stack of the same size and spacing as the Mugen would present is reduced just enough
by those 4 gaps between the 5 stacks so that it does not require a high pressure/speed fan to get airflow across most of its fins. But I do think the Mugen has higher impedance than say the Ninja or HR02; iirc, it benefited quite a bit from a second fan.