With second strand of yarn bind off center 5 work to end. My question is: how do I add the new yarn? Usually I work both old and new yarn together for a couple of stitches and I’m off, but I need to keep the old yarn for the first half. I’m stumped. Can someone help me?
You bind off the center 5 stitches with the new ball of yarn. You will be working the shoulders and each shoulder needs the yarn so both yarns are needed.
Thanks, but the question is how do I add the yarn.
i wonder if this helps - http://blog.berroco.com/2013/07/16/emily-explains-binding-off-in-the-middle-of-a-row/
it specifically addresses 2 balls, with a mid-row bind off. it’s for a sweater pattern, but the directions/idea should be the same…
other places i found online (including an old KH.com forum discussion - http://www.knittinghelp.com/forum/showthread.php?t=96163) said to “Just leave a tail so you can weave it in later, and cast-on as usual.” i couldn’t find any complaints with doing it this way, other than it feeling a little sloppy at first, and needing to maybe tighten up a few stitches before weaving in the end at that point in your project.
hope that helps… - X 
You can drop the first strand of yarn and start the second strand by holding ~6inch tail in your left hand along with the lefthand needle. That’ll anchor it for the first few sts of the bind off and then you can drop the tail and continue.
If you like overlapping two yarns for a few sts (and i use this method, too), overlap on the last 2 sts before the bind off. Then you have both strands of yarn at the beginning of the bind off. Bind off with one strand and leave the other in place.