To 'join' another end of yarn, just leave a tail long enough to weave in later and begin knitting with it. You use 2 yarns and work each shoulder separately with them.
Work across 17 sts on the first shoulder, drop that yarn, pick up the new one and BO 11, then finish the row. Turn and do the WS row, dropping the yarn when you get to the gap and pick up the other one to finish the row. Turn and on this RS row, work to 2 or 3 sts from the gap, do a dec and drop the yarn. Pick up the other yarn, do the first st or not and dec 1 st then work to the end of the row. That's the decrease done once. Work the WS row, then dec on the next RS row, do the WS row and then the next RS row has decs. That's decreased 3 sts at both sides of the neck edge and there's 14 sts on each shoulder. Then you just work across the remaining stitches until the armholes are 4½" long and BO on the RS row.