I'm not sure it is genetic quite the way you are thinking. When hens get excess protein, they can release an extra yolk. This can lead to double yolked eggs or sometimes two eggs in one day. Sometimes, that second egg can be soft shelled because the shell gland does not have time to make enough material for the second egg. Do these hens also lay more soft shelled eggs than normal?
Instead of it being a straight genetic propensity to lay double yolked eggs, maybe it is more because genetically they process food more efficiently?
Instead of it being a straight genetic propensity to lay double yolked eggs, maybe it is more because genetically they process food more efficiently?