Deuces Are WildG 40

Alright, you can either earn this achievement by playing every song with a buddy, or, you could turn on the no-fail cheat and play every song by yourself.

1. Select "Options", "Cheats", "Enter New Cheat" from the Main Menu.
2. Strum each note or chord to unlock "No Fail" Mode.
3. Pressing G from the "Cheats" Menu will disable this cheat.

G + R, B, G + R, G + Y, B, G + Y, R + Y, O, R + Y, G + Y, Y, G + Y, G + R

Make sure you play a few notes on each instrument, for good measure.

EDIT: You should probably include to go to Multiplayer > Co-op in the guide considering half the commenters are confused about this.
For what it's worth, I did NOT have to do this in one session, I did NOT have to hit any notes on EITHER controller, and I did NOT have to keep both players on the same difficulty (The Elven Jedi's solution, while confusing, is correct). I did have to turn the cheat off and fail a song -- this last step should be in the guide, not just the comments.
Posted by Mental Knight 5