1. open the image.
2. drag a guild across the image, it will snap at the center point. (either vertical or horizontal)
3, use rectangular marquee tool to select any half of the images.
4, hit cltr C, it will copy this selection into clip board
5, hit cltr N, open a new doc (don't need to key the images size, as it will open same size of your selection)
6, hit cltr V, it will paste haft of the images in the new doc, flatten, save and close the new doc.
7, repeat step 3 to step 6, select the other half of the image.