Sorry to be a downer, but that's not really good news. There's good reasons why double-yolker layers are culled, not bred, and double-yolker eggs are consumed, not brooded. It basically never ends well. It's generally a heritable trait which risks the life of the hens and the chicks almost never survive.
If it's not too far along, personally I'd throw it, sorry...
But I understand that may not be an option for you. If you decide to go ahead with it you will likely need to be present to assist it with hatching and have to remove one or both chicks to rear by hand as they will almost certainly be far weaker than normal chicks as well as unable to hatch naturally, since that's generally how it goes, and one of them will probably be extremely weak, with an emaciated body, far too weak to cope without serious care, and even then probably too weak to survive beyond a week or two.
Best wishes.