I call it "playing split-screen"
There is no way you cannot cheat t some extent and there is no way you can police it, just accept the fact that you know the other person's sight and they know yours. And if you don't like that get another console and TV then set up a LAN game!
Either that or play a co-op game where it is a BENEFIT to see each other's perspective, or a competitive game with the same screen-perspective like Mortal Kombat.