Isn't Twisted Metal a split screen game though? I'm guessing all of the games in which you either race or drive some type of vehilce, the co-op or multiplayer would be split screen.
I'm trying to think of games in which 2 or more people can play simultaneously, and it doesn't have to go to switch screen. Similar to a game like Marvel Ultimate Alliance today. There are tons of sports games and fighter games in which 2 people can play and the screen doesn't have to split, but I'm trying to think of more action games.
Regarding Loaded: That is an awesome game! I remember that one. One of the earlier PS1 games. I know it was an earlier release, because it was one of those PS1 games that came in a longbox before they switched up to the jewel case style. I know it had 2 player, but can't remember if it went split screen, or if it did it Marvel Ultimate Alliance style. Since it had the overhead perspective, I think it might have done the zoom out thing, rather than the split screen, but I'll have to check on that later today.
Remember Contra on the old Nintendo, how it was a 2D platforming game in which you and a friend could play simultaneously? I wish the PS1 had some games like that, but I honestly can't think of any off hand. One of the primary reasons we didn't see many games like that on the PS1, is because nobody wanted to release a 2D game back then. Sony even had a policy in the USA to actually discourage publishers from releasing 2D games. They wanted to keep the focus on 3D.