There’s no difference in the CO – or anything else, actually. Once they’re cast on, you’ll see two strands in each stitch, but they’ll segregate themselves into pairs that you can just treat as one stitch. The only ting you have to watch out for is making sure that you’re working [I]both[/I] strands together. Where that’s been bothersome for me is:
Increases and decreases – especially k2togs.
Leaving a strand “behind” when wrapping the yarn for a stitch. This isn’t catastrophic, but it’ll leave a loop hanging loose that you won’t know where it came from when you get back to it on the other side.
Occasionally, leaving a strand behind on a stitch, which shows up on the next row as an “extra” strand. Sometimes (if you can figure out which one is the “extra” one) you can simply pass it over and it’ll go where it belongs. If I can’t figure out which one is supposed to be passed over, I usually just knit it together with the current stitch and horse on.
If you knit each strand in a pair individually you’ll increase by a stitch. I’ve done this on purpose before because it makes an increase that’s virtually undetectable. But if you’re NOT doing it on purpose, that means you’ll have no idea where the “mystery stitch” came from.
You’ll see all of this better once you’ve cast on. I’ve done as many as 5 strands at a time and even with my experience level it wasn’t as scary as it sounds.