Screen-peeking is an important meta-game aspect of the split screen fps. Those who don't utilize the ability to look at their opponents screen are crippling themselves and missing out on a very good opportunity to Troll their friends.
Guy 1: You are totally looking at my screen!?
Guy 2: Problem?
I equate screen peeking to forming a strong deck for competitions when it comes to card games, The composition of your armies for Warhammer 40K, as well as hundreds of other things that I can copy paste from the Tvtropes entry on Meta-gaming.