How to add a second monitor

[belahcene <belahcene@iap-cu.dz>]

Hi every body,
I want to connect a second monitor to my PC, I have 2 video card. The 
system detects the 2 cards, but only one monitor is active. I want to 
active them at same time and receive same view at same time.
I saw that  with the Sun computers, Is it possible with linux.
thank you