I don't know about backyard chickens but after working on an R&D Farm for one of the big poultry producers in the country, I can tell you that they cull double yolked eggs in the hatchery because they say it can't hatch out. There isn't enough room or yolk inside for two chicks to develop...
If it's a boy, you may have a very important peice of genetics there. Also finder beware: Tyson chicks come from a very long line of birds that are never outside, they are born, grow up and never see the light of day (other than lamps), so have no imune system to speak of. I'm sure it can be...
We had one hatch on Friday night with the same thing. She's able to eat and poop no problem, so I'm not really worried about it. Is your bird pooping? We had 12 eggs incubating and so far she's the ony one whe's pipped and hatched.... very very disappointing!!
We set 12 easter eggers on Dec 30, so am hopeful for tonight or tomorrow. however, we've been unable to candle most of these eggs because the shells are incredibly thick! No amount of candling has netted us any happy answers.
We also have 12 BCM in the incubator with incredibly thick shells...