Simple current mathematics
Ok I can't resist "piping in" as it were.
You don't need two separate systems to get water to flow at different rates through a single closed system, and here is why:
If your pump moves 50 gallons of water per hour at its 1/2" outlet, then 50 gallons of water will travel through this opening at a certain current, not how _much_ per hour, but how _far_ per hour, velocity or speed of the water.
Agreed? ok next:
Lets say you have a section of tube that holds 50 gallons. How about 10, 5-gallon buckets all connected. Lets say this is 20 feet long. Then for your pump to move 50 gallons per hour, the water travels through the bucket-tube at 20 feet per hour.
At the other extreme:
Let's say you had 50 gallons worth of 3/8 inch aquarium tubing. It would take around 8750 feet of this tubing. That would mean your pump would move its 50 gallons of water at 8750 feet per hour.
Get the picture?
What I am saying is that all you need to do is vary the volume, or more specifically the cross-sectional surface area, of your circuit to vary the speed. You don't need a new design with multiple pumps and reservoirs.
Make a smaller diameter path for you waterblock and a larger one for your radiator, and you will be all set.
Next time we will talk about the temperature variations caused by the speeding up and slowing down of your liquid.
BJL