According to the sources I asked about that very same question; series is mostly better than parrallel. (some exceptions apply)
Reason: resistance differences.
Take my system for example: I have all 3/8" parts (except my radiator which is 1/2") and I am cooling a CPU waterblock and a chipset waterblock using the same pump. IF I were to put the two blocks in parrallel, one block my have a smaller resistance on the waterflow and thus that block would get more volume. (Water is like electricity; it takes the shortest/simplist path).
Again back to my setup; say my CPU waterblock has more resistance than my chipset waterblock (less turns). That would mean (in theory, of course
that the chipset waterblock would get more waterflow than my CPU waterblock. NOW that would not be ideal because my chipset (EVEN with a pelter) gives off no more than 50W where my CPU is currently giving off more like 100+W and my CPU would get MUCH hotter due to less waterflow.
If I remember correctly, you are planing on useing two IDENTICAL waterblocks to cool two IDENTICAL CPUs using the same pump. In that situation IMHO sounds like parrallel would not be such a bad idea since the resistance SHOULD be the same (probably should check that) BUT add any other component (say GPU cooler) put that in series.
Hope that this helps you out.
Good luck.
