What you're doing here is separating the two shoulders. Just knit across the first stitches with the strands you're using. Then, using two more strands, bind off those center 8 stitches and knit across the remaining stitches.
You don't to do anything special to join them. Just drop the two strands you've been using and start binding off those center stitches with the new strands. The binding off of the stitches will secure it.
Now you will have separate strands for each shoulder to work them higher than the back of the neck. Working them both at the same time means to do one side with whatever decreases you have to do for a row, and then doing the other side; turn and work back on one, then the other, etc.