Have seen claims of a double yolker hatching, but, never saw proof! Its usually some one on a poultry message board making the claim??
Considering the millions of eggs hatched each year, there are bound to be freaks/monsters hatch, many do not survive, some survive for a short time and some live quite a while. The same thing happens in all classes of animals, look at co-joined humans, et cetera.

This is confusing to me because I always thought the yolk was consumed by the chick before hatching and that is why they don't need food for 2/3 days. Now I am really curious!

Thats basicly true. Some chicks will hatch with the yol not entirely taken into their bodies, but the yolk is their nourishment which can "feed" the chiock for a day or 2. I have seen eggs where 2 embryos have begun development but they died in shell, never making hatch. The oddities can happen just as often in single celled eggs. When considering all of the possibilites of what could go wrong with births/hatches, its amazing things work as well as they do! As an animal breeder of some 5o years I have seen some strange things, and would add that most have been the results of total outcrosses. I practice quite a bit of inbreeding in all species and rearely have seen oddities from this type breedings, I usually know the parentage of the animals so have(some) control over results. I say this as I kind of figured that inbreeding would come up in the topic.

We had a duck hatch out from a double yolked egg. It had four legs. The main body was female, the embedded twin was male. So I had a four legged duck that was both male and female.

I sold it to a duck rescue organization from North Carolina or Florida, called Lame Duck Rescue. They got a vet to operate on it to remove the male twin, because it was going to cripple the main duck.

So double yolked eggs do hatch out, sometimes.

We put a double yoker in the incubator just to see what would happen. in the end we had 2 fully formed chicks but the never made it out of the shell. Too bad i would have loved to tell people i had twin chickens..Jaime