Double Knitting

From BluWiki

Double knitting is the process of knitting a tube on two needles going back and forth. The trick is to knit the first stitch, them move the yarn to the front and slip the next stitch. Then knit the third stitch. Move the yarn to the front and knit the fifth stitch. Effectively you knit every other stitch. Then on the way back you knit the stitches that you slipped the firs time and slip the ones that you knit. You go back and forth this way until your piece is as long as you wish.

The video showing how to do this is here.