This is a better discussion for the 'egg laying' or 'incubation' forums on the site, not the 'meat birds' threads.
https://www.backyardchickens.com/f/18/chicken-behaviors-and-egglaying
https://www.backyardchickens.com/f/5/incubating-hatching-eggs
It is fairly common in young pullets to lay multiple yolk eggs till their reproductive system gets the kinks ironed out.
As for incubating - DON"T DO IT!
That is unless you want to witness embryos and chicks struggle to survive and fail. Twinning in egg layers is a death sentence for the embryos. Not enough space or nutrition in an egg for two birds.
Egg layers aren't the same as mammals or other live birthers. Mammals can get all the nutrition they need from the mother and an abdomen can swell to accommodate multiple births. An egg has all the space and food it will ever have.
Hundreds of people have tried this and the success rate is usually about 2% for both to survive and less than 20% for one to survive.
Another outcome some don't even consider is that you may be passing down genetics that contribute to future generations laying multiple yolk eggs - which is a problem for survival.
That's why they're so uncommon. They wouldn't survive in nature and are naturally culled, eliminating those genetics.
