Actually, it's pretty simple...
(btw, I have to laugh at the "20% improvement" figure, it should have simply read 1.5C improvement)
The fans we use are not designed to be used in tandem, like that. There is a gap between the fan blades, and the fan housing. Normally it's not a concern when used as a single fan, unless there's a lot of resistance (look at it this way: why can't you inflate a balloon with a fan?).
In tandem, fans are setup to create additional air flow, but the result is also a higher pressure at the fan outlet, which leaks right back out the prop/housing.
By isolating the fans, this pressure loss is minimized.
A fix would be to use what I refer to as turbo fans, where the fan blades are encased in a ring, which sits deep in the fan housing. These are also known as ducted fans.
It would not be wise to try to mod a fan, as the accumulated dust can bring one to a grinding halt. It's far better to just get the right fan.
Personally, I'm not going to use a regular fan on my rig, I'd rather use the cross-flow type, because they are more compact (for the same power), more powerfull, and quieter than the fan types. I don't understand how ya'll don't use these, they're just perfect for a radiator.
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