Except in a few scenarios, I think
single loop is the better option.
In addition to getting better CPU temps when CPU is loaded the most (i.e., when the GPU isn't loaded) and better GPU temps when the GPU is loaded (i.e., when the CPU isn't fully loaded), you also get higher flowrates and pump redundancy (if you're going to do dual loops, you need two pumps...might as well put those pumps in serial in a single loop).
Of course there are exceptions to the above depending on radiator and heatload balance, but people often forget that you also get the cooling power of both radiators when you go to a single loop, not just the additional heat of the other processor(s).