bind off 1 st at each neck dege every other row 4 times- 16 for each shoulder.
The numbers you've posted don't compute. Four BOs from 23 would give you 19 sts on each shoulder.
I don't know how to include the neck part.
What do you mean by 'include the neck part'?
Once you've joined the second skein, BO the neck sts and worked to end, work WS of the one shoulder (you're at neck). Drop that yarn, pick up the yarn for the other shoulder, BO 1 st (neck edge) and work to end. Work the RS of that shoulder to neck. Drop the yarn and pick up yarn for other shoulder, BO 1, work to end of row. Repeat this process 4x (as patt states). You can either measure the armhole to match the back.... or count the rows from the base of the armhole (where shaping started) so that both pieces match.