Guitar Hero: Aerosmith

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

To add to the previous solution, both players must be on the same difficulty and each controller has to hit at least 1 note for it to count. then back out after you finish the last song in the last setlist, disable the cheat, then re-enter co-op and fail any song.

This has to be done in co-op. Many have doubted whether or not this achievement is obtainable over Xbox Live, and after trying this method myself, I can say that it IS possible to get over Live, but, and I stress this, ONLY THE HOST WILL GET THE ACHIEVEMENT.

I was unsure as to whether the achievement would work or not, but I set up a few sessions as a few friends swore they got it this way. Anyway, the boosting method is as follows:
- Whomever is going for the achievement first, should set up a private co-op player match, 7 songs. Send the other player an invite, then when they're in, the host chooses the first 4 songs, the other player the next 3. Complete the songs, then repeat 3 more times. Once 28 songs are complete (4x7 song setlists), set up a 3 song match, with the host choosing the first song and a random one, and the other selecting the last song. The achievement will unlock for the host after all 30 songs have been completed.

This method will net you both Love Me Two Times and Deuces Are Wild in ~12hrs of play, as 4 playthroughs of the set-list are needed (2 per person, per instrument).

I'd like to thank Stealther GodX and Ultima22 for helping confirm this method, although to their annoyance as it ended up with them not receiving the achievement first time through.

EDIT - According to EmDeeGee, the reverse was the case for him, and the achievement unlocked for him without being the host, while the host did NOT unlock the achievement, using this method. I cannot comment on this, as I have not experienced it, but it appears that it may be even more glitch than first thought. So, to clarify, it might unlock for either the host or the other player. If you have any experience regarding the matter, please comment so we can know better the situation.

Benchem's solution actually works as I have tried it, but I'd like to add on to benchem's solution. You don't need to do 4 playthroughs to get both achievements for both players, a buddy of mine and I have discovered that after the host gets his/her achievement, the player joining has to go to career, select expert and select the first song. Once the song has loaded up, hit a few notes, then fail the song, their achievement for playing guitar or bass will pop, saving a lot of time. So only 2 playthroughs are needed, try it if going for benchem's method, happy achievement hunting!