I've done double knitting on potholders and I did a long tail cast on with both colors together. It took a little bit of extra time because I made sure the two colors alternated with each other on the cast on row.
I also cast on ONE MORE stitch than the pattern calls for, then on the first row slip the first stitch (just one color) as if to knit, then start your pattern or chart. When you get to the end of the row there will be one stitch left which you should Purl with both colors together. On the second row (and all the rest) the 1st stitch you will slip will have both colors in it. This will give a nice looking "chain stitch" edge out of both colors.
This edge technique works for flat double knitting but not in the round since there aren't 'edges' in the round.
Also take a look at Amy's video on double knitting. I think it's in Advanced Techniques, and it's also in the pattern section with the dishcloths and kitchen items.
Double knitting is pretty challenging at first, but it gets easier really quickly
One other thing that Amy mentions also. If you need to work to a particular gauge be SURE to swatch because double knitting is often much looser than regular. I usually have to go down three or four sizes in needles from what would be expected for a given yarn. Also when you get to the bind off row you'll be binding off alternately with the different colors in the color that was last used to knit that stitch and you should use a needle at least two sizes smaller than what you used to knit the item for the bindoff row, otherwise it will be REALLY loose and sloppy looking.
Good luck and Happy Knitting :XX: :XX: