No. Decibels do not "add" that way. Two fans which produce 20 dB each would produce, lets say, 25 dB together.
Saound is a wave. Two different sound waves (from the fans) do not combine into a third wave with twice the loudness.
http://en.wikipedia.org/wiki/Interference_(wave_propagation)

Additionally, to add on to marijanovic's point, fans are not the only components that contribute to the system's overall ambient noise. Rotating media such as hard drives are a factor among many others.

Thanks for the reply, noone i asked knew... Soo thanks

Go buy a fan controller if concerned about noise.

Please post more helpful comments. His question was not about reducing fan noise, even though that may be the underlining objective for him.

It has come to my attention (after deleting a handful of nonconstructive posts) that the user is posting for the sake of fulfilling the 10 post requirement to enter the 3GM contest. :/