@0451 I get around 15-20 C delta T. This is high but this is compute rig so the typical scenario is 100% load. CPU overclocking (4790K) is thermally limited because it is running intense AVX/FMA code. I don't overclock GPUs because their power delivery is already stressed by this mode, and I want them to last. Those are Titan Blacks and they get harder to replace.
Since the flow rate anywhere in a loop is exactly the same it doesn't matter where you put a second pump.
If you add 2 pumps in series you will (almost) double the pressure in the tube immediately after the 2nd pump. The restrictions will step down this pressure by a fraction of the pressure before the restriction at each step.
If you space out the pumps the pressure will have a lower maximum and hence smaller pressure drops through each component so a higher minimum. The minimum pressure will determine the flow rate of the system.
Putting the pumps in Parallel will (almost) double the flow rate whilst maintaining the original pressure.
Normally the pressure drop through a water-cooling loop makes this point moot since it's pretty low, but with an immensely complex loop with very low flow, it could be important.
One way around the issue is to mount identical components in parallel.