Blatant cheating. I only play free for all on split screen, unless it's system link team games or something.
1 on 1? ONLY system link. To hell with split screen for that!
The people I play with keep their eyes on their own screen as much as they can though.
Glimpses man. We all have it. You can always tell what gun they have, whether they are camo or not, maybe even a rough idea where they are from the movement, it can be pretty hard to completely ignore the other screen, depending on the TV's size.