I haven’t made this vest, but your instructions say knit 42,
slip 2 stitches onto holder, join another ball of yarn and knit to end. Work both sides at once.
You are going to be working both sides of the front to the shoulders at the same time following the shaping directions they give you next. Right now you knit the 42 and then slip the next 2 stitches to a holder (this could be a safety pin, a bit of yarn or other kind of stitch holder, they will tell you what to do with these later). With the yarn you knit the 42 stitches with you will shape the neck and any armhole shaping they may have going, on that side of the 2 stitches you slipped to the holder. Then take another ball of yarn and do the other side of the front.
They are suggesting you do them both at the same time. That means that you immediately start the second yarn after you slip the 2 stitches to the holder and knit on across them, (probably also 42 stitches). Then turn and work back across those stitches and then leave that yarn and pick up the yarn you left there on the other side and work on across that side, turn and work back, again leaving the yarn on that side of the middle, take up the other yarn and work that side. You will be working the shaping for the neck on each side as given.
I often choose not to work the sides at the same time, but just work one and then go back and work the other. I sometimes get myself mixed up doing the sides at the same time , but hypothetically it should be easier because you can keep the sides even. If you find that to be the case, definitely do them that way, if you are like me you might prefer not to. Just follow your pattern carefully for the shaping and repeat it mirrored on the second side.