Analysis
Under typical computer use, the above test data suggests as a general rule that users would not benefit from setting up dedicated loops for CPU and GPU. Serializing pumps in the same loop also adds a redundancy factor that dedicated loops cannot provide. With superior reliability and lower temperatures at both CPU and GPU levels, single loops appear to win hands down.
Under extreme computer use, this setup recorded a notable advantage at the CPU temperature level for the dual loop, counterbalanced by the opposite effect at the GPU level. This extreme environment uncovered the critical importance of the respective load ratios generated by CPU class devices vs. GPU class devices, relative to the heat exchangers to which they are connected. Clearly, a CPU generating 150 Watts solely dedicated to a triple radiator will cool substantially better than when mixed with another 400 watts generated by two GPU's even with a second dual radiator in the loop. Jedi Masters would say, "we need to bring balance to the force here", and they would be right.
Conclusions:
Under extreme performance scenarios, and from a pure performance standpoint, dual loops versus single loop are neither better nor worse, under the strict condition that the load ratios are evenly balanced.
Under the most commonly encountered loads though, single loops do win.
Under both of the above use scenarios, single loops also win from a reliability standpoint because of pump redundancy.