Double yolk "twin" chicks aren't any different than siblings from different eggs. Because Identical twins form when a zygote is produced, divides, stops, and then continues as two different organisms. But in chickens the egg is fertilized, and put inside of an egg with a yolk. A double is caused when 2 separate zygotes are put into the same egg. In actuality, the fact that chickens are openly polygamous, means that there is a good chance that the two chicks could look different or even have different fathers. Really all you are doing is creating possible disappointment for yourself. But I do wish you luck and hope that the chicks live through.