I hate to be the bearer of bad news but i have experienced this a few times. They come from "double yolkers" or eggs with two yolks. This can usually be avoided by not incubating any abnormally large eggs. The chicks in question rarely survive, and when they do they usually have serious heath problems.There is simply not enough nutrients, or space, in these eggs for two chicks. I would be prepared to invest some serious time in the chicks, if they hatch, and/or be prepared to put them down if they suffer. Again sorry for the bad news.