Don't split the flow three ways, keep it going from one waterblock to the next. If you're concerned about it getting too hot on the CPU, then flowing to the video cards, I'd suggest throwing a second radiator inbetween, usually just a single 120mm, then a 240mm or bigger as the flow leaves the video cards. Another option is to create a second loop, with it's own individual reservoir, pump, and radiator, but that adds more expense.

 

Hopefully DBZ and or Linked Cube will find this post, but in the mean time I will second CrisisDog and say that to cool two cards , the minimum rad you will want dedicated (or added) will be a 240mm x 50mm rad. I would prefer it be in a separate loop., I am undertaking a major water-cooling in my machine right now as well, and found a 360mm x 50mm loop for the CPU , and Dual 240mm for the GPU's (I am running 4 GPU's) in a separate loop to be my solution..
As I said DBZ and Linked Cube are the resident experts and hopefully will weigh in

 

lol experts... it's water cooling a PC not manually calibrating the software on the hadron collider

thread is 3 weeks old? figure OP has found his solution elsewhere...

 

1) Hadron collider ? who the hell said that? it was a suggestion about cooling capacity (BTW, I have seen some rather misconfigured cooling systems)
2) Who claimed to be expert?
3) Three weeks old....I'll give ya that one.