The following is an opinion:
If you run in parallel then you take away the velocity of the water. If you run in series you maintain your greatest velocity at the cpu and then have good flow to your gpu. Remember your gpu doesn't require near as much cooling as your cpu, and your water temps will not be too much hotter after leaving your cpu and maintains a large level of cooling capacity at 2nd stage.
Imagine if you were using a garden hose and wanted to create a spray. If you cut the flow by 1/2 then you won't have the pressure to create as powerful a spray. Besides, if one block is a higher resistance, then the water will seek an alternate path of least resistance this may leave it impossible to calculate efficiency later on.
Thats the way I see it. Feel free to correct me if im wrong.
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