Help with neck shape

I’m a new member and I hope you can help me. I’m working on this free pattern:
I am at the point where I need to shape the neck on the back piece, here is what the pattern says:

Continue in Color Stripe Pattern across 13 (15, 17, 20, 24) sts. Join a second ball of yarn and bind off center 24 (26, 26, 28, 28) sts, work across remaining 13 (15, 17, 20, 24) sts. Working both sides at once, with separate balls of yarn, work in pattern until piece measures 11 (12, 13, 14, 15) in. (28 (30.5, 33, 35.5, 38) cm) from beg. Bind off.

I understand that I am trying to create a situation where I am knitting on both sides with an opening in the middle, I just don’t quite get how to do it. I understand to continue the pattern across 20 sts. But where am I supposed to join the second ball? If I join it before I bind off center stitches, won’t it be carried with me to the other side? Do I first bind off center sts then work across remaining, then when I get back to the first side join a second ball? I seem to be very confused I hope someone can explain this in a simple step by step way.

Thanks in advance!

Join really means just take the new yarn and start knitting with it, leaving a 3-4" tail to be worked in later. So knit the first shoulder, drop that yarn, pick up the new end and BO the center sts and continue across to the other shoulder. Turn and work the WS row with that same yarn until you get to the neck stitches, drop it and pick up the first yarn and work the other shoulder. Turn and repeat.

Hope that helps.

Wow that totally makes so much sense and seems so simple, great explanation.