Remember: being in series wasn't my only qualifier, the more important part of my qualification that you forgot to mention was the counterrotating fans part.
You said:
"Or, at the very minimum, use twice as much power."
Similar to the half egg idea this is an effort to improve air flow/noise, which by definition would reduce power draw/CFM. If you think about it, a setup like ive described would push/pull the other fan it's proper running direction, so power would be less than with the 2 fans standing alone since each has part of it's work being done by the other. When both fans of this configuration are powered up there is load sharing.
As you can imagine air coming from a fan is not flowing from the fan in a direction parallel to the axis of rotation (which the fan blades spin around). I'm guessing the pattern would be more like a whirlpool effect. With an ideal (non-existent, and perfect) fan the air would flow parallel to the axis of rotation, and the fan blades would not interfere with its path. My belief is that the counter rotating assembly would help to rectify these issues I've pointed out. The issues in this paragraph are the reasons to argue for this fan configuration.
you said:
"2 25cm fans in series will still only move 25cm... you can't get more air out than you put in."
While it is certainly true that you will never get more out than goes in, you could make each fan push more air by making it operate more efficiently. If something you are doing by putting them in series is allowing them to run more efficiently, then you WILL get increased airflow through both of them. I'm not even saying that putting 2 REGULAR fans in series will incrwease airflow, but my SPECIAL set of fans would (if I am correct).
I just wish I had the money to do a prototype so I could prove/disprove my hypothesis.