The co-op achievements in this game are a bit on the funny side. First, you must have an XBL session going to be able to co-op. There is no split-screen or LAN co-op available. Second, the developers didn't add a minimum player limit, so you could host a game with just yourself in it and get the co-op achievements. Third, you don't actually have to play an entire level, just reach the end of it. This means you can start at the final checkpoint of a level and just run it, assuming you've already reached the end of one of the levels.
From the main menu:
1. Campaign -> Co-op Campaign -> Host a Game
2. Change Privacy to Invite, Change Scoring to On (so you can also get the other co-op cheev), Create Game
3. Select Checkpoint 1-10: Dark Energon (assuming you already beat Chapter 1, any other final chapter will do as well), Select your character
4. Start the game and head to the end. A final score screen should pop up after the chapter is complete and your achievements should unlock.