Just to add my 2 pen'th . I realise that the person who started this thread is probably dead, or has at least worked it out for him, or her, self by now but...
I had ordered a fan too many (I'm sure we all know that feeling) and rather than shove it in a draw, I had the same idea as ramsfan6239.
Being lazy I thought I'd google it before putting in any effort, which lead me to this thread.
But, like many so threads it didn't provide an answer; or rather it provided many contradictory answers, all equally convincing, stated with authority backed up by impeccable professional references.
I decided a practical test was the only way to know.
The fans were two akasa 120/140 vipers, running outside the case. I gauged the air flow of a single fan by dangling the end of a roll of 3" wide paper down into its exhaust and seeing how high strip was lifted. I thought I might have to set up some sort of grid behind the paper to make it judging the lift a little less subjective. In the end I didn't need to. The fan ran as close to silent as made no difference, and the paper lifted to just over 45 degrees.
I placed the second fan directly behind the first and slowly closed the gap between them. As the fans came close to touching the noise started and the paper in the exhaust stream drooped. The noise was most noticeable (though never loud) when the gap between the fans was completely closed; at which point the paper merely flickered in the slight draft.
So there you have it. Not very scientific, no static pressure sensors or theories of aero-dynamics but the proof of the pudding, as they say, is in the eating.
Also if anyone reads this: what's happened to all the original replies to this question? There were dozens and dozens, honest. Or I did I just dream them? I posted my answer then couldn't (and can't) figure out why I couldn't see them any more.
I'm new to Tom's and this was my first post. Did I do something wrong, have I erased them, or was my answer so brilliant that the moderator's thought no other reply was worth keeping and took them off?
Any answers? Aybody?