[krazy]

I used to have a pair of fans stacked together in my chimney fan (call it a blowhole if you prefer) primarily for the purpose of being able to shut one of them off (it was a bright blue LED fan that annoyed me at night). The second fan screwed onto the bottom would still send plenty of air through, and I was pleasantly suprised by just how much of an increase having both of them on at once would make.

I have to agree with MMZ that placing some sort of shroud between the two fans to separate them a half inch even would probably make them noticably quieter.