There will be a one-row "lag" in the decrease on one side, but, believe me, it will not show. Complete your separation row - you now have two balls of yarn attached to your work and 26 stitches cast off for the neck. Turn your work, and purl the 24 left shoulder stitches. Pick up the other ball of yarn, cast off 3 sts on the right neck and purl to end. Turn, knit all 21 stitches on the right shoulder; pick up the other ball of yarn, and cast off 3 stitches on the left neck and purl to end. You now have 21 stitches on each shoulder, and after working 3 rows even (no increases or decreases) you are done.

Thanks a bunch, Anne. I think I've got it. I was afraid the difference in rows would be too obvious. I think I've worked on this forever and I want it to look somewhat symmetric.