12v Rails on my crossfire R9 290 Setup

So I've read some conflicting information. I'm gonna be running 2 r9 290's and my brother told me that you need to use 2 different 12v rails from your PSU to support ONE video card. My PSU has a 12v rail that branches off into 2 parts to connect to the video card. Isn't that enough? He's saying it can bottle neck but I don't believe him. Can anyone shed some light?