In Resistance: Fall of Man, the Single Player campaign could be played both one player, and co-operatively with a friend locally with a split-screen. This has been scrapped for Resistance 2.
So, if you want to play through Resistance 2's single player campaign with a buddy... Tough luck! Just play the co-op campaign with eight buddies online instead and be happy with that.
Note: The Co-Op Campaign will have split-screen play, it's just the single player campaign that won't.
This isn't really that big a deal, just that some people might be interested to find out.