You have to bind off at the beginning of a row, which is 'each edge' of the piece. For a neck that's being worked both shoulders at the same time, the row split, so you have 2 'beg of rows', it looks something like this:
vvvvvvv|____|vvvvvv - so you would BO at the beginning of a RS row, work to the neck, take the other yarn and BO sts at the neck edge and finish the row. Turn and BO at the beg of the WS row, work to the neck, take the other yarn and BO sts on the other neck edge then finish the row. Each shoulder has its own yarn end.
BTW, is this the same pattern, or another one?
sue- knitting heretic