Yes, a single DDC pump with top is really a heck of alot of pumping power and alot more than most systems even need. There have been a couple of people that have mentioned to me that they actually got better results running a D5 at lower settings, and that's all because of heat dump. Two pumps usually only gains you about maybe 30% more flow. So if you had 1.2 GPM two pumps might give you 1.5GPM. The thermal efficiency gain is probably less than .5C, yet you may raise your water temperature by that much or more depending on your radiator setup.
But in the end I think it really depends on the block. There's not a whole lot of data out there like C/W vs flow rate that well defines what the gains are. D-Tek and Swiftech are the only ones that publish this sort of data.
Data aside, there have been a mixture of results. Naeku has tons of experience with high pressure pumps like the Iwaki RD-30 and has reported up to 2C worth of gain. McCoffee noted that during dual D5 testing that he really couldn't measure any gain between one D5 and two.
With all of this combined, I think two pump "Could" provide some gain, but it's likely not going to add up to more than 1 or 2C at most and could potentially make it worse depending on your radiator.
I got the impression you were interested in a multiple block loop so you'll probably be running some decent heat loads on the radiator.
But hey, you know, this is all extreme systems stuff, running two pumps fits right in with that. Plus you get the added benefit of redundancy, if one pump fails, you've got an automatic backup. Just don't expect to see some big 5C gain from flow rates, it can't happen.
Like anything in watercooling siginficant gains only come from an accumulation of many minor ones. To do that you have to tinker as there is no absolute best setup for any one system.
Enjoy the new build, and post lots of pics, and invest in plenty of time afterwards doing the best thing you can for any sytem and tinker!!

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