In my post about neat shoulders I mentioned using short rows shape shoulders and joining the seam using a three needle cast off.
When you shape shoulders using short rows you end up with a set of stitches on a holder rather than cast off stitches. You can join them using the steps below.
2. Hold the pieces you want to join with right sides together. You want
the two sets of stitches to line up on the parallel needles.
3. Insert your needle into a stitch from each needle. Using a third needle
of the correct size, insert the tip into the first first stitch on the front needle
and then the first stitch on the back needle.
5. Repeat steps 3 and 4 and cast off one stitch. Work the next two front
and back stitches together in the same way and then pass the back stitch
on the right hand needle over the front one as you would in a normal cast
off. Continue to cast off the pairs of stitches all along your seam.
(Here you can see the row of green cast off stitches along the seam line.)
Here you can see the final result in the Lily sweater.