BeQuiet Silentwings 3 are very quiet and apart from looking a whole lot better than the Noctua, will have better affects on the motherboard. One of the advantages of aircoolers is bleed air, that's air that escapes out the sides of the heatsink and helps keep the VRM's and other voltage regulatory circuitry surrounding the heatsink cool. Important for OC or heavy use purposes. The NF-F12 is a focus flow design, great for radiators as the air cone is very tight, almost 90°. This makes for maximum air through the heatsink, very little bleed. Adding a second fan will drop cpu temps Mebe 2-3°C under loads, but is even worse for bleed air as the 2nd fan draw keeps it centered in the heatsink.
A better fan for that cooler, from Noctua, would be the NF-P12 or the new NF-A12X25, which is just as silent, but isn't focus flow, so has a broader cone.
And I'd still not use a second fan as either of the Noctua will outperform the stock CM 212 fan.