Double yokes are very random (and quite normal) for young birds - especially sex-links. Consistent double-yokes may be an indication of obesity but this is pretty uncommon.
Isn't cracking open a "double-yoker" good luck??
The good news is you really don't have to "keep them warm". They are built for the cold. Frankly, my flock of 30 (Red and Black sex-links and GP Hamburgs) seem to have greater hardship in the hot Minnesota summers than they do in the bitter Minnesota winters. That said, I think the best thing...