Always post a link if you have it when asking questions. Is this the pattern?
You don't need to join a ball for that first side. Just keep knitting that one side after you've cast off the 12 sts. You will need to join a ball for the other side of the front.
What I would do is - knit across the 12 stitches the put them on a stitch holder, then cast off the middle 12 and continue to do that side and bind off. Then pick up the stitches from the holder and attach a new ball and finish that the same as you did the other side then bind off.
You can do both sides at the same time with two balls as the previous poster mentioned as well if you prefer.