OK, let's tackle the first question of using two pumps. When you put two pumps in series (assuming identical pumps for now), the "P" portion of the P-Q curves basically add to one another. Two pumps can lift water twice as high as a single pump, yet the maximum total flow you could ever develop is unchanged.
What this generally means for a closed system is that flow rate will increase, though not by a factor of two. What this means for temperatures largely depends on how you were doing with a single pump. You'll be dumping much more energy into the fluid with the second pump, but this added energy will be mitigated by a greater flowrate to carry it. You'll also have a lower delta-T from CPU to water, though by how much is the crux of the issue.
For this you need to estimate where you fall on the block's °C/W curve vs flow rate. At zero flow, °C/W is practically infinite. As flow begins, the thermal resistance repidly drops. As flow increases yet more, the incremental benefit in °C/W shrinks. There is a sort of "knee" where you transition from rapid benefit to rapidly diminishing benefit. If one pump doesn't get you over the knee, the second pump may. Then again, if one pump doesn't get you there, a single larger pump would.
The delta-P vs flow data is there for many block's from Bill's data. The same information is available for many radiators. You can put a pretty good estimate on it for tubing vs flowrate and length. Do your best to estimate your final flow rate based upon the P-Q curve of the pump and your components. To estimate it with two pumps, double the "P" for a given "Q" on the pump's curve.
Doing this will get you in the ballpark, but the only real proof is in actual operation. Know this: You can always make changes and it's best to start simple. Get things going with a single pump and iron out any bugs. Based on how that performs, you should be able to predict very accurately how things will go with a second pump.
Almost forgot, the pumps don't care about the arrangement details so long as the minimum pressure remains high enough to avoid cavitation. I recommend people orient things based on "good" tubing runs and "good" placement of components (like getting the coolest air to the radiator regardless of whether it falls "ahead" or "behind" the CPU).
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