Chained shoulders

On the spot:
Leave the shoulder stitches on the needle. Place the back and front stitches wrong sides together.
With a 3rd needle, knit together the 1st stitch on the front needle with the 1st stitch on the back needle.
Repeat this operation with the 2nd stitch on each needle.
You have 2 stitches on the right needle. Cast off the first of these 2 stitches over the second stitch.
Continue this way until all the shoulder stitches are cast off.
You get a visible chain on the right side and a shoulder seam that will not deform.

On the reverse side:
To achieve an invisible seam on the right side, place the back and front right sides together.
Follow the instructions above.