With double knitting, one basically knits a two-sided piece of fabric...so for example, a scarf with the 'smooth' side of stockinette showing on both sides. One way to do this would be to knit two stockinette scarves and then sew them together. But I did it with double knitting.
Basically, you cast on with a two colors, alternating them (on this scarf, it is purple and pink). Then it is knit with color A, purl with color B across one row, and then knit with color B and then purl with color A back. After a few rows, the stitches arrange themselves so you can see that one side is purple and the other side is pink.
I didn't do a good job of explaining it, and in fact it is all kind of magical. I'm using Lucy Neatby's bubble scarf pattern ( here and apparently I forgot to do the ribbing at the start, oh well) and she explains the process very well.
Yarns: kauni effectgarn for one side, rowan calmer for the pink other side.
This one should be fun to watch. Stay tuned.