Ah the dual setup... I remember doing this for my aquarium!
The surefire way to do this (and most expensive) would be to put a Y splitter (as stated above) and a flow meter at the 2 ends of the Y. whichever one is flowing faster, you put a restricting valve on it, and adjust it until the flow is the same.
Now let's get back to the real world!
As long as your blocks are exactly the same, that your tubing is exactly the same length, and that the tubing is in a relatively same position, orientation AND shape, you can assume that the flow will be pretty much the same.
You can run them in series, really. As long as you've got a decent flow, you'll probably never notice any difference (scary, isn't it?).
The plus with a dual-CPU setup, is that the flow is restricted, which is something that the rad likes.
What pump are you going to run?
Are you going to be using a thermal monitoring software?