Stretch a game across monitors?

[Wolfmight]

How do you stretch a 3D game across multiple monitors? I have two that share the same desktop now, but I can't figure out how to stretch a game across them. I just want to see what it's like.

[harrycary]

If you have a separate graphics card, the software for that card(i.e. Nvidia, ATI) may allow you to do this.

(mobos with onboard graphics don't have multiple monitor capability)