It would not work.
1: Coolant from the central res would be free to flow through the high speed line feeding the block. There is nothing preventing the hot exhaust fluid from immediately reentering the high speed loop.
2: In order to prevent this occurrance, you would need a radiator inline after the water block to guarantee some cooling to the fluid in that loop. Otherwise, you risk a high probability of system failure.
3: Because of the prior cooling from the radiator inline with the water block, the rad in the slow loop would be much less efficient since radiators are more efficient at higher temperature differentials. Using a standard radiator in this loop would probably just absorb heat from the pump.
SOLUTION:
Use a large (think in the order of 2x3 FEET like on the front of a F-Series pickup ... expect to pay $3-400 for a new high quality one) radiator inline with the second pump as a COOLING RESIVOIR. It would serve two purposes: it would increase the coolant volume and further cool (albeit slowly) the coolant. No fan would be required to use this unit due to its size (and absolutely massive surface area) and it could be bolted to the side of your case. The hard part would be adapting the inlet/outlet to the size of hose you're using, though that could be done through PVC fittings, I believe.
Here is a diagram:
Note, the yellow is the central res (note the baffles to prevent direct recirculation), the light blue are the rads (note size diff
), the red is the block, and the green is the pumps.