From the book “Commercial chicken meat and egg production” (Donald D. Bell, William Daniel Weaver, Mack O. North):
“Normally, only one yolk is ovulated per day, but occasionally two may be released and on rare occasions there may be three. If two are ovulated at the same time, normally only one enters the oviduct, but if both are picked up simultaneously by the oviduct, a double yolk egg will result. About ⅔ of the double-yolk eggs are the result of ovulations within three hours of each other. If there is a great difference in ovulation time, two eggs may be produced on the same day, but usually the second is soft-shelled.”