A Cop-Op or Versus mode?
What made SH So scary (For me, then again I'm a Pansy Growing Pansy who get's thrilled by paint drying) was that the monsters weren't humanoid (With the exception of a few) and didn't have persoanlities, they were parts of the character's flaws and past, having the monsters player controlled or having a partner kind off defeats the whole porpuse of the concept.
Oh well, if it's well executed, it's gonna be great, if it's terrible, I'm going to throw all my SH games in the trash since in the current state that the series is in, It's hanging by a thread.
EDIT : Look at what Happened to RE5. A partner/Companion defeats the whole porpuse of Silent Hill, Silent Hill is basically like being locked in a small room with something that hates your guts. Intriguing, Interesting, and Scary. Being Locked in a small room with something that "Likes" you kind of defeats the whole point.
And since there's 2 players, there's gonna have to be more weapons around, defeating the whole pointof the "Frantic-Hard To get Resources-Scary" Nature that Surival Horror introduced to Action games.
SH + Multiplayer = Big flaily mess that defeats the Whole point of the entire Franchise.