Penelope consists of a front and a back piece. The front piece is always cut-on-fold. For the back piece, this depends on whether you wanted a vent, or moved the zipper to the back seam. Either of those options will have you cut out two individual pieces for the back.
When you cut out a piece on the fold, remember that the fold does not need to be in the middle of the fabric. Choosing the placement of your pieces wisely can greatly reduce the amount of fabric you need for your skirt.