how to use two graphics cards? (Ubuntu 13.10)

Hello,

I have a laptop with two graphics card (SLI). GeForce GT 650M x 2.
I use blender for video editing, compositing. And everytime I render, it’s always only one of them (the left side one) works really hard! It gets sometimes very hot, and I’m starting to worry a bit. Is there a way to separate the render task on both cards? the left side one’s temperature goes 80% and it’ll be nice if I can do 40% for each card…

If it’s not possible or too complicated, I’m thinking on removing one of them and install SSD in the place instead. Becuase all I do for heavy graphics is blender compositing, and I won’t need 2nd card if it can not be used by blender…

I have installed Nvdia driver and it works generally fine. (with X driver from ubuntu, it was all glitch and I wasn’t able to do anything with blender)

'sorry for not being able to be technically precise, I’m quite new in CG and hardware stuff…

thanks.

and I forgot to mention that I activated on preference option CUDA, and ‘GeForce GT 650M (2x)’ already. it obviously make things a bit faster, but still only left one works socks off while right one sleeps.