What I would do is just put a valve to restrict flow on the Northbidge line just before the rejoining of the two lines.
It would seem to be a easier way to restrict flow than trying to compute some thoretical tubign size/line length/water block restriction type of solution.
Personally, I just plumbed the whole stinkin mess in series, and I still get good results, bu I could see why you would want it that way (
http://forums.procooling.com/vbb/sho...&threadid=5188).