Do you mean where you knit across one shoulder, bind off (or put on holders) the middle stitches and knit the other shoulder?
Once you take the middle stitches out of the row, either by binding or holding, you need two stands of yarn--one for each shoulder.
If you are binding off the stitches, you can knit the first shoulder, let that yarn hang there for the next row back. Then you attach a new ball--just start knitting with it or attach it as you would any new ball of yarn. You use that ball of yarn to bind off the middle stitches and work to the end of the row. Now you have a separate strand of yarn at each shoulder.
If you are putting the stitches in the middle on a holder, you attach a new ball of yarn to the second shoulder where the neck stitches begin.