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.