With the same number of components, a single loop will out perform a dual loop as heat from the components under stress is lost through all the radiators, where as in dual loops obviously some of your blocks are isolated from some of your radiators. The highest efficiency you can aim for in a dual loop when distributing radiators is to give a proportionate amount of radiator to the amount of heat dumped in the loop. If by a huge co-incidence that the radiator distribution matches the heat dumped (which varies depending on varying stress) in the same ratio, than the efficiency will be that of a single loop of all those components. It goes without saying that the higher the heat (in Watts) needed to be distributed by the radiators in that loop, the higher the equilibrium temp on the cores of the components.
Here is an example of a simple typical single loop and double loop
Single Loop:
One CPU 140W
Two GPU 240W
480mm worth of radiator
So 660W is being more or less evenly distributed between 480mm of radiator as water temperature is more or less a constant through the loop (especially between radiators). So 165W is being lost through each 120mm of radiator when temperature of components have leveled out.
In a dual loop with a 360 and the GPU's in one loop, and the 120 and CPU in the other.
In the GPU loop 520W is distributed among 360mm of rad, so 173w per 120mm of radiator, while the CPU loop sits at a lower 140w. So the CPU sits cooler but both GPUs sit hotter.
There is no advantage in this configuration. People argue that it allows you to clock certain components higher for benching but synthetic benchmarks usually only put specific components being benched under stress, so if i were to run a CPU benchmark, the GPUs would stay idle and the single loop set up would provide a whopping 480mm for just CPU cooling, while the dual loop still only has 120mm, as it is isolated with the other 240 going to waste as the components in that loop are idle.
In gaming, different components are in varying degrees of stress depending on whats going on, on screen. In this situation, single loops are obviously more efficient at distributing heat, as single loops cater better for the hotter component, which will be the most important one (the one under most stress).
