Actually I am going to be building an external waterbox to make stuff a bit easier and quieter. It will just be a black opaque plexiglass enclosure housing a 120.3 + 120.2, 2x Laing D4 pumps, a 200W mATX psu for power, a fan controller, a reservoir, fill port, and a bunch of fans.
Going to be a bit expensive as I have to buy another pump (used d4s are cheap though, no one likes them ), some more fans, fan controller, and the plexiglass..but I think it will be worth it as once it's set up it will make swapping hardware and stuff easier. It will go under my desk and my tower will go on top of my desk.
So once that's all up and running adding another video card to the loop is perfectly do-able.