Comments on the flow rate - depends on the pump and the blocks. Yes, you split the flow in ~half when going parallel, but you also cut the flow resistance by
almost half as well, meaning your pump's flow rate will go up with them in parallel. What the net result is regarding flow depends upon how restrictive the blocks are and where you are on the PV curve of the pump.
General comments as far as temps :
Pelts + CPU = significantly diffferent system from only a CPU.
Q = h * DeltaT (heat flux equals heat transfer coefficient times the temperature delta)
End result - there is no "one size fits all"
If you are going with 1 pump and rad, try both ways and see which works best for
your system configuration like MT did.