First of all...
As for the egg sitch' it would be too much work then worth, it is VERY unlikely that you can hatch 2 healthy chicks form double yolkers, this member was very lucky...
When my ducks lay double yolkers I don't even check if they are fertile, and I know it's not fair for the possibility of healthy chick(s) but the chance of everything going fine is far too low.
Did your brothers friend hatch any from the double yolkers? 2 chicks or 1? I guess if his hen is laying healthy double yolkers that hatch often then he must have a good hen, the chance would far better of everything going OK, but for normal hens a double yolker is un-ordinary let alone hatching something or somethings from them...
Just out of curiosity, would you hatch them in an incubator or under a broody hen?
I am assuming that he is hatching them, I have yet to see them, so IDK, just that he will replace any egg that is not a double yolker.
At the moment I have enuf problems with normal eggs and incubation, I myself would not try a double yolk egg (at this point), and I have not got a single double yolker
Congrats on the hatch how are they doing???? PICS!!!!