You will be binding off the centre neck stitches. Attaching the second ball of yarn means you can leave your working yarn where it is, bo with the 2nd ball, then continue with that ball and do the k8 on the last 8 stitches. Now you will be working both sides of the shoulder at once, and to do this is actually really simple. Once you've done the K8, you just turn your needle and work back across the 8 stitches, drop that ball of yarn, then move across to the other 8, (which should have the yarn ready to work with, as we dropped it at the end of those sts in the previous row), and work them. Turn and work back on both 8 sts again, (for the work 2 rows even bit), then at the end of this row, you'll turn and bind off both of the 8 sts. Ta Da... you've done both shoulders at once.
Hope this helps.