This is one where you should probably do some experimentation, but here are a couple things to consider:
First, stacking fans will not double airflow, a 36CFM fan is just that. Stack 10 of them and you've still got 36CFM. However, when you stack fans you do increase pressure.
Second, Nexus fans are designed to be quiet case fans, they aren't designed to blow air through a radiator. As a result, when they're hooked up to a radiator they may not be powerful enough to actually blow air through it in sufficient quantities to adequately cool your system. So, having them stacked may provide a real benefit because by increasing the pressure you may get reasonable airflow.
Third, when you stack fans you greatly increase turbulence. So stacking them isn't (or shouldn't be) just a matter of bolting them together. I'd advise doing some research on this so that you can minimize the turbulence and actually get the air to go where you need it.
Lastly, push/pull is generally much easier to do than is push/push, but with the Nexus fans you may actually need the push/push configuration. I'd try the push/pull first just because it's easier and see what kind of temps you get. If you're satisfied, you're done. But, if you're not, then either consider different fans or researching how to use them properly in a push/push configuration.
Oh, one more thing, the amount of air pressure you'll need depends in large part on the radiator you're using. The thicker the radiator, the more air pressure you'll need to get air through it. So, using a radiator like a BIP would require much less air pressure than would a BIX. If you're going for near silent operation, look into getting a open single-pass radiator. The Nexus fans would actually work pretty well on one of those, though I doubt it would help much if you're trying for monster overclocks.
