your first idea wont work (wont work as well). May even decrease perfromance as the pressure drop available accross the system is less so flow rate will decrease. Also water might just end up being circulated through your rad instead of being pumped around the system.
While water flow rate will not effect rad performance alot it will effect waterblock performance. Running two pumps in seris will double the maximum head but maximum flow rate will remain the same (as an example this is rougly equivlant to the step from a hydor l20 to a mcp600 here
http://www.procooling.com/articles/h...haestus__1.php) . Two pumps in parallel will double the flow rate but max head will remain the same. So if your running a restrictive system seris is better (i think this would cover most water cooling loops) and via versa for a high flow system.
It doesnt matter where you put the pumps in the system the same amount of water will flow through the system as water is incompressible and mass flow rate is conserved. The one caveat to this is that pumps add heat to the water so it maybe better to add them to "Hot" parts of the loop ie not before the cpu block.
edit: above stuff is an ideal appoxiamation there are loads of little problems which means that some of it might not be true for a real pump loop but the results will be very close.
An easy way (depending on how you think) is that hydraulic systems are like electrical ones (if you squint and ignore some problems). Voltage is pressure, flow rate is amps. Resistance is a restriction causing a pressure drop (water block = lightbulb?) and pumps can be thought of like batteries.
I think overvolting a pump is probably a bad idea.