You work on both shoulders with separate strands of yarn. You can work across one to the bound off sts, drop that yarn and pick up the other strand and work the other shoulder. It's the same row, just split.
At the same time means you're going to shape both the armhole edge and the neck edge differently, but over the same rows. For the shoulders, you can only bind off at the beg of a row. If you BO at the end of the row you end up with the yarn at the edge and a gap over the bound off sts so you couldn't knit them with the same yarn.