Yeah the basic story is that if you put two fans inline -> A -> B -> then CFM is only a little improved.
The more complicated story is that doing that helps with a pressure differential. If there's a pressure drop right behind B, then B would be moving a much reduced quantity of air. However with -> A -> B ->, then A helps take care of the pressure drop between A and B.
In other words, if you have 'stuff' in the airflow between "in" and "out" then having a fan on "in" AND a fan on "out" helps get CFM back to where it should be. Of course this is a very common factor in computer cases!
Just to substantiate this, I've tried putting fans inline (almost right after each other) and it helps very little. (Also of course if A and B are within a few cm, then you may get a lot of wind noise.)
But having both an intake and an exhaust fan does help - some. Not like doubling your airflow.
the wesson