Most people will not put them in an incubator because the chances of them hatching is extremely low. You were very lucky in this situation.
What normally happens is one chick is at each end. When the first chick zips the shell it uses it legs to push the top off the shell to "hatch". In a double yoked shell it cannot get the leverage to push the top off as it's feet are pushing against it's soft and fluffy twin. A horrible way to die. Take care in who you tell as most poultry people consider it cruel to set these eggs. I'm very glad to hear that it worked out for you. Those chicks must be extremely strong and hearty, the perfect addition to a backyard flock.
Reducing our carbon footprint one meal at a time.
Romeldale sheep, poultry, and Silver Fox Rabbits.
@pancho: Simple, they are in an incubator and we were able to watch the hatch. Besides, we have three chicks and only two broken eggs!
You might inspect the other eggs. Some times an egg will look like it hasn't hatched as the top part moves back into place after the chick hatches. Also sometimes one egg will get caught up in the other egg after the chicks hatch and look like one egg.
I am not saying it didn't happen but if it did it would be the first time it has
Since this is your first time hatching eggs it is much more likely one of the other things happened.
If you watched the video you would see the woman taking the chicks out of the egg.
Chicks will develop in a double yolker egg but won't hatch without help.
When a bird hatches they will pip a hole in the egg then make a complete circle around the egg breaking the shell. The top opens and the chick pushes its way out. When there are two chicks in the same egg they do not have the room to turn a complete circle to break the shell. They will both die. Each chick is also attached to the yoke sack by a cord. When two chicks try to turn in the same shell their cords become entangled preventing them from turning.
A double yoke egg will develope two chicks but, like I said, they will not hatch. They can be taken from the shell.
Look back at the video. At the end the two chicks are laying side by side. Look closely, notice anything strange?
I have never heard of them being able to hatch on their own. The only ones I have heard of had to be helped out of the shell, including the video.
Double yolk eggs are larger than single yolk eggs. They also should be obvious when candeling.
One bloodline I have lays mostly double yokers. Customers love it, but a pain to find hatchable eggs from them.
I used to know a woman who went to a layer house and they gave her all of the double yolk eggs. She had a route she sold only double yolk eggs to. She had a good story why her hens only layed double yolk eggs.
I've never seen it personally but, If they can develop all the way there have had to be some hatch out. Here is an article extract from Oxford Journals. Can't get the whole article without being member.
Basically it says from 208 double yolked eggs, 64% were early dead, 31.6% developed to day 14 or beyond, 2 eggs pipped and 1 egg produced 2 viable chicks.
I don't know why its not POSSIBLE to happen without human aid, though I would say it a rare occurrence.
If anything, IMO (and yes, I'm familiar with the video) if it could happen WITH a humans intervention, then I'd say it has a pretty good shot at happening without human intervention, (which is nearly always interference anyway) in barnyards across America and beyond without us knowing.
Again, not saying it's common, but as demonstrated by the video - possible. As they say, "life finds a way". I'm leaning toward the 'it happened' on this thread.
(to the O.P. Don't expect it to ever happen again though!)