A few days ago Debra asked me to discuss purling with two colors. My recommendation is DON'T. Just kidding, I know there are times you have to do it. I would avoid at all cost an entire colorwork project that was knit flat. That is just insane. Not only do you have to deal with working a color pattern chart from both directions but you also have to deal with making sure the same yarn is held above the other yarn to keep your project looking good. (Some folks call this yarn dominance.)
Many Dale of Norway stranded sweaters instruct you to knit back and forth after you start the neck opening when you get near the shoulders. Some knitters will try to avoid this by adding neck steeks. Honestly the only time I purl with two colors is if I'm doing a striped sock heel.
You hold the yarns for purling with two colors just as you do with knitting with two colors. You either hold both yarns in your right hand, one yarn in each hand, or both yarns in your left hand. Even though I'm a two-handed two-color knitter, I prefer to purl with two colors with both yarns in my right hand. It is just faster that way for me. I hold the background color wrapped around my index finger and the foreground/contrast color wrapped around my middle finger. Then I just kind of flick the appropriate finger when it is time to purl. Here's a photo from Stranded Color Knitting that shows the finger position I use when purling with 2 colors. (Thanks again for the photo Chris!)
