Adding fans won't do too much unless you're maxing out the stock fan... all it will do is make the stock fan slow down due to pwm. If you add fans, it cools the boards and the temps drop and the stock fan will slow down until the temps rise to 40^o again.

If your fan speed has increased since adding the second fan then it sounds like it's causing more blockage than increased air flow.

If the chips reach 50^o and the stock fan is running at over 3000rpm then it would be time to add fans to keep it down to 40⁰ again.

I've added fans to my overclocked unit, but that's only to keep it quiet and I've added them on the outside blowing straight on the last 3/4 of the chips, it seems to produce better results  

EDIT: P.S. if you want to drop the temps, then just remove the blue wire from the fan connection as mentioned further above and watch the temps drop to 30^o as the fans spins up to crazy speeds