If, for example, you have an image that is cropped to 20x30 at a resolution of 300 DPI and you want to split it into 2 - 20x15 prints, open a new 20x15 canvas at 300 DPI and use the marquis tool to select half of the original image and drop it into your new canvas. Crop the original 20x30 to a 20x15 with the other half of the image. Remember to make a copy of the original image before you start. For Gallery Wraps, you will need to take the sides of the wrap into account and for metal prints you will need to take the bleed into account.