I had thought about stacking the radiators in my case due to a lack of space, but saw some reviews on stacking radiators, and decided against.
I was trying to blow air across the top of my case (see sig), as that was the only real space for radiators. I eventually did stack them (sort of). They're bolted together, far apart, with opposing fans, and the top fan in my case pulling out. I have High RPM 120x38mm Panaflo fans in push (on fan controller), with 120x25mm 1900 RPM Scythe fans in pull (on controller, too). So I pull in from the sides (fresh air from outside the case), and exhaust out the back. So far it's worked well. My rear exhaust fan is a 14CM Noctua. The fan doesn't have the greatest static pressure, but is quiet, and if fed some positive pressure (and how in my case, with 15 intake fans, and 2 exhaust), they can really move.
Total fans for my system (for those who doubt the 15), I have 4x92mm blowing on the motherboard, 3x92mm on hard drives, and 8x120 set up for the radiators, and 2x120 for exhaust.
From what I've seen, pushing fans will give a slight edge on performance vs. pulling.
seems to be pretty decent. I went with a push/pull non-shrouded setup (space limitations). I don't have to run the fans at full speed, but I can if need be. Just some food for thought.