You can’t bind off at the end of a row–you get a gap. I guess you’ve learned that, huh?
Always bind off at the beginning of the row at the side where you are working. For example, when you are at the beginning of the neck edge, that’s where you bind off. You won’t be able to bind off on the same rows on each side, but that’s ok–it’s not possible.
Working both sides at one, bind off 3 sts each neck edge 2 times…
So let’s say your at the point where you bound off the center ten and finished the row.
You’ll have to purl across the first side without binding off anything, bind off three on the neck edge of the other side and finish the row. You’ll then knit across the side you’re at, the bind off the first 3 at the other side and finish the row.
More simply put, bind off at each neck edge when it is at the beginning of the row for that side. Do it 3 stitches, and the next time it’s at the beginning, bind off 3 again.
Bind offs are easy to take out, so just slowly pop out the yarn from where you bound off at the end and put the stitches back on the needle and purl them.