It looks like on average we see a delta T of 3 degrees C between 1 fan and 2 fans. On the other hand, you have 5-6 dB increase in noise output. So essentially a fairly small decrease in temperature for a doubling in noise output.
I am not seeing that in the above data... but I am having some trouble sorting through it.
I have always assumed that two fans running slower should obtain the same amount of cooling as one fan running faster... and do so with less noise. For example two Nexus running at 500rpm compared to one running at 1000 rpm. It seems to me that the two slow fans would cool quieter and cool better than the one fast fan. But admittedly I have no data to support this.
My assumption may well be wrong. But to prove or disprove it I would like to see a comparison between (a) one Nexus and (b) dual Nexus run at various speeds and compared. I don't believe that kind of data is up in the table above, nor do I believe that data is available from SPCR testing.
If I am wrong I would be pleased if someone would be kind enough to politely correct me. I find it difficult to believe that MikeC has never made any subjective comments on this subject. Perhaps someone knows where he might have commented on it and provide a link to it. Or even where there is some clear data one way or the other (preferably from SPCR... I just don't trust sound related data from other sites).