You will work whatever # of stitches it asks for the start that row, then leave that yarn attached, and attach ANOTHER ball of yarn at the same place. Use the NEW ball of yarn and proceed to bind off the center stitches. Then continue on, working to the end of the row. Turn it around and purl back (or whatever your wrong side row instructions are). When you get to the middle stitches that are bound off, you will then move back over to the first ball of yarn and work the wrong side of the other side of the V-neck. It will make SO MUCH SENSE once you start doing it. I hope that my description has been helpful.
To sum it up in a nutshell, you will have a seperate ball of yarn for each side of the neck, and you just happen to be working them on the same needle, even though at this point they are really seperate entitites (ie. the two sides will never again be joined)