How can I (if I even can) login two users to play a game online. Specifically Call of Duty 4...

I mean play online, in a split screen from the same PS3 on the same t.v.

I know xbox does it, I don't know why this wouldn't.

I have looked around for an answer for awhile but haven't found anything, sorry if this has been covered before...Any help would be greatly appreciated.

Thanks