I was messing around with two colours of yarn recently and started a pot stand pattern in the Tech Knits book. It's a double strand cast on so you basically just cast on stitches as normal but holding two colours of thread together as if they were one. Note: The cast on stitches need to alternate in colour. I cast on 70 stitches in total.
Row 1: Knit the first stitch which is, say green, using only the green yarn to wrap the needle. Bring both the green and blue yarn to the front as if preparing to purl, then purl the next stitch (blue) using only the blue yarn. Move the working yarns to the back of the work again is if preparing to knit, then knit the next stitch (green) using only the green yarn. Keep doing this to the last stitch, which you should purl.
For Row 2: Do the same but reverse the stitches i.e. Purl the knits and knit the Purls.
Edit: Keep repeating rows one and two until you're ready to cast off.