if you want two fans on it - i would do one push, one pull. But with a heatsink like this - it wouldn't matter adding that second fan. If anything, i would just get just one fast fan on it. To make a heat sink cool down, you want the maximum amount of air flowing through it as fast as possible. By placing a fan on the other side of the heatsink, you could possibly be restricting some airflow on parts of the heatsink (even though it's pulling air out of it). You will probably need to do some trial and error testing to see whats best. I will say though, stacking fan on top of each other doesn't do anything
I would use that second fan as an exhaust fan for your case.. get that heat out of there!