Multiplayer/Co-Op & Frictional Games? NO!!!

[Mehis]

If you look from the developers perspective, it's very hard to create scares that includes both players. There can no longer be self opening doors, Shadow appearing or just those scary ambient sounds that scares you, if the other guy is at the other side of the map. And when there is somebody yelling on the mic, it's going to be even more horrible.
Those already takes big part of what Amnesia is about.

Able to kill your friend is a horrible idea. I don't see anything immersive or realistic aspect on it. It would be more like Trollnesia where you troll your co-op buddy just to kill him.

I must say co-op Amnesia sounds pretty fun experience, but it doesn't really fit into Amnesia itself.