here is a link to my You Tube tutorialfor ONE way to cast on in two colors for double knitting..
there are may ways..
1--cast on provisionally, in color A (main color) Use a cirulcar needle as waste yarn!
knit X rows of stocking knit. (if desired, work 1 row of purls on front for 'turning row' at the half way point. (1/2 of X) )
fold work in half, (with stocking knit side out.)
hold work similar to 3 needle bind off.
(if you didn't use circ as waste yarn, undo provisional cast on, and place stitches on a needle)
pick up second yarn..
[B]Knit stitch 1 from front needle with Yarn A.
Purl stitch 1 from back needle with Yarn B
[/B]repeat this across rows now you have 2 sets of stitches, plus a stocking knit hem.
--other ways have you cast on with yarn B between each knit stitch.
it really depends on the effect you want..
i use my 2 color cast on because with potholders, its a fast easy edge.
(PS--if you use the same cast on with a single ball of yarn, you can cast on in knit and purl.. --sample shown is 1 X1 ribbing.. but you could do 2 of normal long tail, then 2 backwards long tail and have 2 X 2 ribbing in your cast on row!)