Well the water wont be that hot at all because the water flow is quite quick so the one section of water is not able to pick up heat a lot at a time so once it goes to the graphics card(s) it's still fairly cool. You would be able to do this as long as you bough an extra water block compatible with your graphics card. Most water cooling kits will only give you 2 water blocks (1 for the CPU and one for the GPU) so you will have to buy one more. Other than that it should be fine.
