When I cast on for magic loop, I divide my stitches in such a way that the beginning of the round won't fall at the side break between needles. Otherwise, it would get all stretched out and sloppy.
Things are going to get very knittery from this point forward, friend. I'm just warning you. To do this I cast on each mitten separately on different circulars, dividing the stitches evenly for front and back. Then I join a mitten for knitting in the round, hang a marker on the first stitch (Since I know it will be at the side break, I just need to remember which side is the beginning), and work one or two rows in the round. I then repeat the process for the second mitten. When they are both at the same place in pattern, I'd slide one next to the other on a long circular, making sure they mirror each other exactly. I can always use different colored markers to clasp those stitches at the side together so they won't get pulled and look sloppy.
You can see where I used a stitch marker to hold together those stitches at the side where I re-joined after working the thumb hole, just so they won't get too stretched out.
Stitch markers save me when I do socks too. I find I always need to rearrange stitches on each sock when it comes time to work the gusset or a toe. If I'm knitting two at a time, I can't just slide the left sock's stitches over from the back needle to the front because there's a whole other sock's stitches between it's front and back side. So when I'm still at the back of it, I slide the ones I want moved to the front off of the needle and hook a stitch marker through them and let them hang. Then I work around the right sock and when I get to the front of the left one, I can slip those stitches off the hanging stitch marker/ holder and put them to the front of the sock. Fascinating, isn't it? :)