Data on single vs dual loop?

[alacheesu]

I'm sure it's in here somewhere but my search-fu is leting me down (to much noise from .sig files I think). I'm looking for some actual data on single dual-radiator loops vs dual single-radiator loops.

A dual loop is just two single loops, and a single dual-rad loop is of course also just a single loop. Thus knowing how single loops behave should be enough to answer your question.

I gather the consensus is that dual single-radiator loops perform better but I don't really believe it as it doesn't make sense to me (which is not to say it isn't true, only that I can't figure it out). If the total heat output is the same and the cooling capacity of the radiators is the same, why would chopping it up into different loops improve things?

The main reason you run dual loops is to separate the heat. Let's try a small, simplified example:
2 equal rads, each dissipating 300W at 10C.
2 GPU's, dumping a total of 400W heat.
1 CPU, dumping 200W heat.

Everything on one loop would give you an air-water delta of around 10C.
Running 2 loops would give you a delta of 6,7C on the CPU loop and 13,3C on the GPU loop. This is good, because you want the CPU as cool as possible. GPU's usually don't care at all, and the increased temp is most likely irrelevant. If you wanted the same CPU temp in a single loop you would have to add a third rad.