To work both sides separately, you'll need two balls of yarn. The left side of of V will use one ball and the right the other. You can either complete the left side, then complete the right. Or you can alternate by knitting one row on the left side and then knit one row of the right and continue working left, then right. When it says to dec at neck edge, the just means to work a dec st on the edge of the work that will touch the neck. If you're wearing the item, you would dec on the right edge of the left side and the left edge of the right side.
Hope that helps ...
