Then again, I always knit stranded knitting with my right hand only. I just don't like using both. :shrug:
It IS very important, however, to keep one strand on 'top' and one on the 'bottom' to prevent the tangling, as Kime said. Let's say you're using blue and red--you want the result on the back to look like this
What I do is wrap the main color by pinching and wrapping, and knit the contrasting color with it over my pointer, if that makes any sense. If you keep the strands lined up, they'll look better on the back, too.