You need to have either two balls of yarn or a ball that you can pull from both ends.
Put the two ends together and tie a slip knot in the ends, insert needle and CO as usual! Use as much yarn as needed and when you are done, you just cut one of the strings.
Can I get a Woot!??
I guess I should add that I am excited about this because I had to CO 211 stitches and I had to rip it out at least a dozen times because I ran out of tail!