Push-pull with dissimilar fans?

[wagex]

Turbines actually do have similar structures inside 'em, they're a big stack. Moving blades, stationary blades (pointed the other direction), moving blades, stationary blades, etc.
All the moving blades turn the same direction, but the stationary blades kill the rotation of the air between each rotor.

EDIT:
Here's a lovely picture from wikipedia: http://upload.wikimedia.org/wikipedia/commons/5/56/J85_ge_17a_turbojet_engine.jpg

5s that why some deltas have the reverse stationary blades behind them to stop the rotation and increase the static pressure? and also keeping a narrower stream of air?

 

[Bobnova]

Yup, that's it.
Those fans stack pretty well even without a counter-rotating second fan.

 

[macforth]

Well....this reasonably well known CPU cooler is sold with two different fans:

http://www.overclockersclub.com/reviews/noctua_nhd14/