I do double knitting all the time. I do the same cast on as the other two have mentioned.
I also think of a pattern as one sided, but do the stitches as "stitch sets" as described by ecb.
If you are going to knit part of a pattern in single knit and then switch, I would say the easiest way is to just hold both yarns together for the row before you begin the double knit. If you have to have all one color on that row (not the mix you get if you hold two colors together), then use an extra strand of your main color yarn and join the contrasting color after that row, then as you go back the other way pick up the stitches as mentioned above (k1 "red", p1 "black", repeat until end of row). Remember that on your reverse side rows you have to do this in reverse (p1 black, k1 red). I almost always do this for hats knit in the round so I don't have to worry about reverse side rows.
recent FOs: Shedir cap, Bricks of Atlantis, Pirates Hat, Citrine Socklets, Teddy bears, 1 entrelac sock
OTN: scraps blanket, Shedir for Teenie, Ice Serpent Hat
NIL: Qiviuk tam re-visited; weatherproof gloves; sweaters
My knitting blog: http://emilai333.wordpress.com/