You can't Co-Op the Prequel levels in story mode, though you can do it in Free Play. Aside from those two specific levels the entire rest of the game can be played Couch Co-Op. You can unlock everything as well, though getting S rank on stages may be a little difficult since you're splitting kills with co-op.
"I just came out of a PIE, you swooning harpy! That is totally bat-**** insane!" - Mehrunes Dagon