They must have been selected as double-yolkers, or at least been sold as super-size eggs.
I put a double-yolker under a hen once and got the chicks to pipping stage, but sadly both died before hatching fully.
Until a day or so before laying, the hen's eggs are just a cluster of yolks of varying sizes. Each day one yolk sets off down the oviduct and is coated in turn with white, membrane, shell, colour, spots and wax. If two float off together down the oviduct, they can both get incorporated into one egg.
Not sure if there's much genetic effect -- many hens produce the odd one, especially if they lay good big eggs anyway.