What I do is to grab the working yarn near where they come out of both balls. (Actually, you could probably just hold the balls themselves, now that I think about it.) Anyway, I hold the two sides up and let my work hang down like a yoyo (make sure you're not in danger of the socks slipping off the needles while you're doing this) -- the two strands of working yarn will untwist. You can pull the two sides in opposite directions to help the unwinding along if you want. I do this every few rounds. After you get a substantial amount of sock done, you can tuck the balls into their respective socks to keep them apart. It takes some time to get to that point, though.