You'll need a 2nd end of yarn as you'll be working the shoulders as separate pieces with the bound off sts for the neck in the center.
Continue with the decs at the armhole edge, take a new yarn eand (from another skein or the other end of the one you're working from) and bind off the 18 sts in the middle of the row and work to the end of the row. *Turn and work back to the center, drop that yarn, pick up the other, BO 4 sts and finish the row. repeat from *. Then turn and work to the center, drop the yarn, pick up the other and BO 2 sts and work to the end of the row. Repeat the last instruction three more times.