You'll have two different sets of yarn, the original that you knit 24 sts with, and the one you join to BO the center sts (check that it's 21 sts, because you should have the same amount of sts for each shoulder). After you BO then finish the row with the new yarn, turn and work the WS. When you get to the bound off sts, drop that yarn and take the original yarn, BO 2 sts and finish the row. Turn and work the RS, drop the yarn, take the other and BO 2 sts then finish the row. Repeat.
sue- knitting heretic