Since no one was able to provide a concrete answer, I took this to physicsforums.com and asked them. There was some debate there as well, but the end result I got was that two fans definitely can help. The reason is that the second fan increases the pressure along the length of the radiator. One of the regulars there stated this:
The general rule of thumb is that fans in series double the pressure at the same flow and fans in parallel double the flow at the same pressure.
Basically, if the radiator were a straight, frictionless tube with no gaps, then this rule would apply exactly and there would be no benefit to a second fan. However, in the real world, the radiator is far from perfect. There are blockages on the edge of the fins, friction as it flows along the fins, and lots of places for the air to leak out along the way. What the second fan actually accomplishes is not increasing the overall airflow, but making sure that more of the air flows smoothly along the entire thickness of the radiator.
The increased pressure would allow it to keep the speed up by overcoming some of the resistance of friction. That small volume of air that would have hit the first fin and blown out sideways will now be more likely to stay within the radiator, continuing to gather heat along the entire fin. This explains why hardware/overclocking sites and manufacturers report modest, but not phenomenal, gains by adding a second fan in push-pull configuration.
Thanks to everyone here and at Physics Forums who weighed in. It was an enlightening discussion.
(I did go with two fans because I found a great sale on them.)
Best Answer
No.
Fans don't dissipate air, they move air around in the hope of dissipating heat from nearby hot objects using convection (or, in the case of hot sweaty humans, by also increasing evaporative cooling).
What is important is the temperature of the air relative to the temperature of the hot object, the exposed surface area of the hot object and the rate of flow of air across that surface. None of these can be determined by counting fans or by simply considering their location - since nearby surfaces and ducting affect where the air flows to.