when I did it, I had 2 balls of yarn going on at the same time...
I felt I had to do it on straight needles for some reason, but I have been told you can do it on circs..if you choose....but for me the straight needle version.
CO the required number of stitches with ball one
on the same needle, CO the required number of stitches with ball 2, and then just follow the pattern respectively. I put an obnoxious stitch marker between the sleeves, so I'd remember to change the yarn I was working with. It gets a little tangly sometimes, but since the sleeves are narrow....it doesn't take too terribly long to do...plus with premie clothes...you'll be done in a flash.