Good pointsIMO, it's not likely for the double yolk egg to have one embryo absorb the second. (because the 2 embryos would be separated by the distance of the 2 yolks. But, what do I know! The situation being described: a single individual posessing genetic material from 2 distinct sets of DNA is called Chimerism. The individual would be a Chimera.
https://www.medicinenet.com/script/main/art.asp?articlekey=8905
https://www.ranker.com/list/chimera-animals/mariel-loveland
In the case of the roo with comb and wattles growing on the back of his neck, I would guess that he is a chimera! The second link shows a number of animal chimeras including a rooster.