Not 'if you need' the 2nd ball of yarn, you will. Each shoulder is going to be worked with its own ball or end of yarn. If you have 49 sts at this point, then that's exactly right - 18 sts twice plus 13 is 49.
Do the first row exactly as written - k18, take the new yarn end and BO 13, then continuing with that yarn knit the remaining 18 sts. Turn and do the WS over 18 sts, drop the yarn when you get to the neck and pick up the other one, BO 3 sts and finish the row. Turn and work 15 sts to the neck, drop the yarn and pick up the other one, BO 3 and finish the row. You now have 15 sts on each shoulder so, when you turn BO all of them with their own yarn. Or you can do another row of each on the WS and BO on the RS.
sue- knitting heretic