Yes, there are twin yolks, but that doesn't mean there will be two chicks. Many times, both, or neither develop. Rarely, both chicks will develop, but they seem to always need help hatching, just because of size issues. There are a few topics on them in here!
Thanks so much - i have not gotten a double yolk egg - but i had seen a post where someones first egg from their hen was a double yolker - and they showed it in the frying pan - and got me to thinking!