Well that's the thing, both of the yolks can be fertilized and produce two chicks. Unfortunately this often results in two dead embryos, or one early quitter that later make the egg a toxic environment for the second chick, also leading to two dead embryos. In very rare cases two chicks will hatch successfully or one chick will survive. In a worst case scenario, that I have only seen once, the larger chick absorbed the smaller chick along with the yolk sac and surgery had to be performed to remove the still living, kicking and peeping second chick from inside the other. I never did get an update on whether either survived but I've never been able to get that image out of my mind...