IT is considered uncommon for doube yolk eggs to hatch, but as with all things in the animal world, that isnt set in stone.
Much of that comes, I think, from the admonition to not try to incubate doublers. Since for a VERY long time, it was more about doing what worked and not experimenting, the admonition sort of stuck as immutable law. Afte all, when your life depends on your livestock, its best to do what you can rely on.
As we are learning here everyday on BYC, many things can be done that the go against the rules.
SO, in a real sense we're all groundbreaking pioneers!!