Note: I've heard from some sources that this will only work on earlier versions of the disc. I can't confirm or deny it, but if you don't get the achievement, that may be a reason. I have the version that in the guitar bundle, and it worked for me.
Only the lead controller will get this achievement. That being said, 44 songs is a lot for two people to play twice. Also, since you can't get this online, it's can be a bit of a pain to find someone willing to come over to do it.
The good news is that you can get this by yourself.
First, clear your cache. (My Xbox => Memory => Hard Drive, press Y for options, and clear the cache.) All this does is delete your game updates, it doesn't touch your game saves or gamer data. So yes, when you play other games later, you'll have to update them to play them.
You have to clear the cache first because later updates have kept the No Fail cheat from letting you get this achievement.
Second, turn on the No Fail cheat code. Go to Options, Cheats, and enter the following: GR, B, GR, GY, B, GY, RY, O, RY, GY, Y, GY, GR.
Third, set up your two guitars. In fact, you don't even need guitars. Regular controllers will work. Your main tag should be player one (just in case... I'm not sure). Since you're going for the Back Up Hero, make sure that you play EVERY song on bass/rhythm.
Fourth, just go through every song on the list. You don't have to play, just keep pressing the green button to go on to the next song. You also have to play all the songs after the career. You don't need to play the bonus songs, but you do need to play all the unlocked songs.
Fifth, you must turn off the No Fail cheat, and go back into co-op mode. Fail the first song (which lets the game know that the cheats are off). Since all the other songs are complete, it will pop.
Edit: I'm including a link to a video that shows that you don't need to play any of the songs, and all the songs have no score. All credit to d1mebagdarrell, who posted the video.
Edit 2: Per Disturbed 360a, you can get the achievement if you do it all on one sitting, even if it doesn't let you overwrite your save. Thanks for testing that out!