I had a double yolker still moving on like day 18 or 19, but one had died. It looked like there were still active veins and it hadn’t drawn down enough. I left it overnight, and the live one had stopped moving. I opened the egg and there was a dead fully formed but small chick and glob of bloody goo and slime.
I have a hen that lays 2-3 double yolkers per week and I always incubate them hoping for identical twins, but none have hatched so far, almost all develop well until I candle at 9 days, but most are not moving by the time I lock down.