Parallel pumps is not really a good idea, unless you have some kind of one way valve after each pump. If one of them dies, then it is likely that some of the water will go back through the other block loop, without passing through the rad. This would be bad.
However, if you had two pumps in series, and one of them fails, you will still be getting flow through all of the components.
I agree with Blackeagle, run all the flow through the CPU, then split off to the northbridge and the GPU.
8-ball
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