Wire-Spilcing would just lead to reduced power going to the initial default fan, probably making both fans do the equivalent of the Factory default fan
Why not just attach the fan using a 4-pin molex connector to the powersupply, or better yet, use a 3-pin connector to attach it directly to the motherboard. No need for any of this intricate wire-splicing.
One fan on top. One fan on the side. Heatsink will be mostly made of prongs so air can pass freely both ways.