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.
View / New Guide... and then enter for position 50% and select vertical or horizontal.
Do understand that 50% is not exatcly 50% if you try to cut an odd number of pixels in two pieces. For example 301 with 50% will results in 150 and 151 pixels.
Of course you can use this to slice your images in two equal parts too. It shows again how many ways there are in Photoshop to do a single thing, which is also the beauty of this program.