I used to know someone who tried and tried to hatch double yolkers and every time he ended up with 2 dead chicks (or chicks to be). I thought it was horrid when he knew what was going to happen
I forget who it is, but someone on BYC has youtube videos up of hatching a double yolker....she had to help the chicks out, but I think she ended up with two live chicks in the end...but from what I have read, generally, double yolkers don't make it.
Double yolkers have hatched before but the chances are slim. It's always advised and I will do the same - Hatch normal eggs. No over sized, no undersized, no really weird shapes.
If you do a search at the top of this page "Incubate double yolk egg" you will get 5 0r 6 threads here on BYC that have already answered most of your questions.
