Dan,
Do you have the *Multi GPU* option turned on in the video settings by chance? I know there is one in there, but I don't have the game installed so I can't go in and check.
Maybe one of you who have the x2 and COD4 installed can make sure you have that checked while your playing. It will also reset itself every time that you quit a game, so you'll have to enable it when you start the game again.
Only option I see that is similar to what your asking is "Dual Video cards"
In which case I have that set to no, as I only have 1 Video card, does it mean multi GPU's?
I guess Ill try it with it on.