I vaguely remember two fanmates in series don't work. ...
Surely two Fan Mates can be used in series.
If one turns a Fan Mate to the max., it acts as an 1,5V Zener diode - whatever voltage one puts in, out comes (input - 1,5V). E.g. 12V in -> 10,5V out, 5V in -> 3.5V out and so on.
So, by combining one Fan Mate that has been dialed down to min (5V) and another Fan Mate is connected in series, the output voltage will be around 3,5V. This particular combination is rather dull, as better solution would be to power the second Fan Mate from a 5V rail direcly, thus eliminating the other Fan Mate. But using two Fan Mates will enable the use of voltage range between 3,5-5V.
, I haven't played with my CNPS7000's fan, but if the fan is identical to the 92mm Zalman fan one can buy from the store, I can confirm that (at least my sample) it starts just fine with 3,5V (the fan is from CNPS6000). Though I think that the speed would be too slow to cool properly.
The easy solution would be to get more Fan Mates...