Duplicate the image and use the Image\Canvas Size on each one of them. Make sure that you start with an image with an even number of pixels for height or width (depending on whether you want to cut horizontally or vertically).
Let's say an image is 200pixels wide and you want to cut it vertically, then the first image needs to be 100pixels wide with the anchor arrow at 9 o'clock and the other one needs to be 100 pixels wide too, but with the anchor arrow at 3 o'clock.