What you're doing at this point is binding off stitches for the back of the neck and then working both shoulders. They each need they're own ball of yarn, since they're not connected anymore. You leave the yarn for the first shoulder behind, attach a new ball, bind of the stitches and work to the end with the new ball. This way each side has it's own yarn and it is in the right place.
Next you need to follow the instructions in the pattern until the piece measures 14 inches.