As everyone is saying I think a series loop is a better option. The water will eventually reach equilibrium and the cooling rate is more dependent on the amount of dispersion of heat you can acquire (More Rads/Fans). Although here is another example of a parallel circuit.
It pretty much is a dual loop shared between the cpu and the gpu with a shared dual reservoir:
1) Waters pushed up into the big radiator
2) Out to the cpu
3) Into reservoir 2
4) Out and down to small radiator
5) Up and through GPU
6) Back and into reservoir 1
It becomes parallel inside the reservoir since the waters technically shared between reservoir 1 & 2.
A little to get your head wrapped around but it pretty much ensures that an equilibrium will be created between the CPU and GPU over time. It also happens to be my rig ;) But I like sharing ideas so here you go.