Not sure how well that'd work, maybe if you spliced and soldered it'd work, but I'm not entirely sure because it gets a tad confusing when you have bundles of wires. If you're good at soldering give it a try but don't blame me if you end up with two non-working headphones.
However, there are a few adapters on the market which let you just plug in your own headphones and still have headphone controls. [http://www.amazon.com/gp/product/B001C0LBEG?ie=UTF8&tag=rab777hp-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B001C0LBEG|This one] by Griffin works with iPhone as well as iPods, but [http://www.amazon.com/gp/product/B002K4GBEI?ie=UTF8&tag=rab777hp-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B002K4GBEI|this cheaper sleeker one] by Belkin works with all iPod models and looks rather nice.