Hmm. I have some problems with agreeing on the points made here.
I have 2x MCP600, and made several test setups. One simular like yours.
You idea is good.A radiator is restrictive. If you have 2 pumps.. why not use it?
Well. I made several setups, even with only the WW and 2x MCP and a load of water.
Extra speed in the water will give you only 0.5 degree. Sorry but true.
In my current setup (best balance between temp and noise) I only use one pump, the WW, the D-teck Z Chip, Double heatercore, single heatercore and a BIX.
The temps with one extra pump are 0.3 degree lower.
I placed my GPU in a seperate loop, and that did help
Pumps in series will give better head, but, depending on the type of impellor, als more noise.
In my case, with closed impellor, it makes no sence. Yes my speed goes up, but my temp hardly goes down.
Best way to lower your temps is by optimizing your airflow in the radiator(s)
I have my water temp now 1.5 degree above ambient, with 3 fans at 7v
With the fans at 12V it is less then one degree.