I have wound up with over the years with a dual 120mm fan radiator, a single 120mm fan radiator, a Swiftech CPU water block w/220W TEC, a Swiftech GPU water block w/80W TEC, a regular north bridge water block, and a pump.
How I have it setup right now is:
Pump > Big Rad > CPU > Small Rad > GPU > Pump
(I’m not using the north bridge block right now)
My question is if you should use a Y connector and run 2 loops? One for the CPU and one for the GPU. Like this:
Pump > Y connector > Big Rad/Small Rad > CPU/GPU > Y connector > Pump
In all honesty, you would not be increasing your cooling potential any by splitting your loop, and could be introducing more flow restriction instead.
As I see it, you have the most efficient setup now. The only time you should consider splitting your loop is if you have a series of cooling blocks that use a smaller tubing (ie, stepping down from 7/16" to 3/8" or 1/4"), or you have a specific device in that secondary loop that dumps a lot of heat into the system, enough so that it raises the ambient fluid temperature beyond an efficient cooling range for other devices in the loop.
make two individual loops, you wouldn't want one massive one like that, it won't perform as good and since you already have the two rads and pumps, you really have no reason not too, res's are cheap and so is tubing