Double-yolk eggs (two embryos in one egg) are difficult to hatch as there is not enough space to allow two chickens to develop. Most double yolks will start to develop then eventually die. However, rarely a double yolk egg will make it to hatch date and does require specialized attention to get the chicks to survive hatching. Remember, the process of hatching requires very specific conditions. If the temperature is too cold the chick will die. If the air is too dry the chick will be wrapped tightly in the protective membrane. If the air is too moist the chicken may suffocate in the protective membrane.
And if you happen to get both chicks to hatch, they may not have fully formed organs and will die. It is a rare occurrence if both chicks survive.
Most experienced chicken owners will not use a double yolk egg to hatch. But they do make good eating!