It seems pretty strange to me why you would want to run it though the same rad twice, you're still getting the same amount of cooling since you can't make more liquid magically pass though the radiator.
A single in and out circuit will do just fine, if it's possible to route tubing through it twice, then that would just mean your pump would be working harder to get the liquid through, I'm pretty sure you won't see a difference in temps on the CPU and GPU just because you haven't hooked the rad up in between them.