There should be an air cell at the top. The chick should never completely fill the egg or it won't have room to internally pip and get into position to hatch.
It is very rare for a double yolk egg to hatch. They will often develop, but if two complete chicks form, they can't get into position to hatch. sometimes one embryo will absorb the other and you will end up with a chick with extra legs or something if it survives.
Double yolks are fairly...