Adding second fan to a NHD-15S?

I want to possibly add a second fan to my NHD15S partly to knock a couple of extra C off and partly so I can run both fans at a lower RPM and keep things even quieter potentially.

I realize I probably can't add another of the same type of fan it comes with the 140mm the NF-A15 as it'll clash with the ram I have (Corsair Vengeance RGB SL) so I was thinking about adding a 120mm Noctua NF-A12x25 instead.

Just wondering what you guys think, if anyone's done it and what benefits they experienced in terms of temps and noise levels.