First there's the cooling part of things, with a density of approximately 1000kg/m^3 water is around 830 times denser than air at 1.2041 kg/m^3.
Air has a specific heat capacity of approximately 1 kJ/kg/K whilst water at 25 degrees C is about 4.2 kJ/kg/K. Multiply out these factors, and you end up with a cooling performance of around 3500 times the performance of air by volume (factors such as boundary layer thickness and turbulence excluded). The flow rate of the water is going to make a factional difference to you cooling unless you've got a preposterous amount of things in your loop like 4 gpus, 3 rads, cpu, hard drive coolers etc. We are literally talking 3-4 degrees at most, certainly nothing worth worrying about.
Secondly, there's the way pumps perform. Pumps do NOT create pressure. They create flow. Resistance to flow creates pressure. The more resistance, the more pressure at any given flow rate. If your pump is only capable of sustaining a certain pressure differential across its inlet and outlet, as you increase the resistance, the flow rate will decrease until the system balances.
So for dual pumps...
It really depends on your loop order, if you have a huge number of components in series, they represents a very large resistance to flow. Placing two pumps in series will allow the pumps to provide a much greater effort in terms of pressure, as the maximum pressure they can support for a given flow rate is essentially doubled (think of this as simply adding pressure).
Placing two pumps in parallel, the pumps both produce the same volume of flow, and this flow as you would intuitively think adds together. It gives you no advantage in terms of pressure. You would only use this configuration if you had a vast number of components in parallel. And I mean 4 or more upwards before you start to notice a difference.
Of course, what speed you wish to run your pumps at will introduce sensitivity much much earlier. I barely have my DCP4.0 turned on because its a noisy ****er and I HATE noise. Despite my CPU GPU and GPU all being in parallel, I noticed only about a 6 degree rise in my delta temps between this and when it was just my CPU and radiator and my pump running at full pelt because my fans were too noisy to hear it.
Its really very very marginal and essentially, you shouldn't worry about it. but if you do for any reason, and your loop is entirely off the chain, then it might be worth planning you loop and pump config to get the best noise to flow ratio.
You'll have plenty of water either way.