True, but if you drop a rock off a cliff in Colorado and another off a building in Tibet, they both fall down.
I'm just stating general facts about how these things work. It would be a rare system indeed that could prove me wrong, and I'd be VERY interested in seeing it. To make parallel pumps work better than series you would require three things: two identical pumps, a water block with a channel diameter equal to the combined flow from both pumps, and a MASSIVE radiator (think of the main radiator on a F350 or so). Even then, it would probably be a dead heat with the pumps in series with the same components. On the small scale we're dealing with, I dare say it would be impossible for a parallel pump arrangement to give better results.