it depends. if your running 2 identical blocks and pipe runs, thus giving very similar flow resistance you should get about equal cooling from both blocks. if one block has much lower resistance that the other most of the coolant will go via the lower resistance path - even if you are going from say 1/2" to 2*1/4" and have a huge pump performance from the higher resistance loop will always be disapointing. you also tend to lose a lot of flow and pressure going thru generic nylon pipe splitters, tho cutom made ones can mostly stop this.
best used for SLI/Crossfire gfx cards, dual cpu or matching gfx/northbridge blocks (like the koolance 180G blocks) i wouldnt advise mixing say a TDX and a Maze3 as all the flow will go to the maze 3 and the TDX will get almost nothing. ofc, i am willing to be corrected on any of this if anyone knows better. but i thought id answer the bump.