The funny thing is....it's the white that turns into the chick and the yolk that feeds the chick while it's growing inside the egg. I probably wouldn't try to incubate a double yolker, as I have seen chicks hatched with weird used-up yolk-like things in their chest area and they didn't make it. I always wondered if it was the result of a double yolker.
Now that's an interesting question you bring up. If the egg was fertilized twice would it develop into two chicks, one using each yolk? Would two chicks develop in the one shell? If one was of a more scientific type and willing to deal with potentially sad results, it might be interesting to incubate one and see what might happen.
Gee, I can see I was absolutely no help whatsoever! I'm hoping other people chime in on this one!
Jayne