It's true, when someone else is there, it does diffuse some of the tension of a horror situation...of course, you could argue having ONE other person with you can only do so much for your confidence when you're facing a slathering horde of THOUSANDS of twisted abominations from an insane asylum inmate's nightmares, some of them the size of a freight train. But that depends a lot on how immersed the two of you get into the game. If you both talk in calm tones and tense whispers about the game's content, that maintains focus and atmosphere. But if you start shooting the breeze about the Super Bowl or how much the latest Michael Bay film sucks, then that's definitely going to break the mood. Of course, as some have already pointed out, Dead Space hasn't been supremely scary in general; not in the same way as games like some of the Silent Hill franchise or Amnesia: The Dark Descent. I'd call it a good sci fi thriller with its good startling moments, but not often that genuinely SCARY. So adding a co-op really won't do all that much to it, I figure.
Oh, and as for the comic..."Cotton-Eye Joe" isn't bad, but I would have gone with "The Hamster Dance".