Standard "rule" of 120x120mm of rad space per component, 240x120mm if you're going to overclock, thickness plays less of a role in getting temps down than the surface area. As for dual loops vs single, dual is generally more of an aesthetic choice, though some people like the ability to independently control the loops or they really want to minimise the temps of certain component/s. Any well thought out single loop with the right amount of rad space will cool a heavily OCd dual GPU + CPU + mobo VRMs setup quite easily.