You can actually get this achievement without even trying. Just login the main account and a second account. Play the level using the secondary account and just get through it, dying or not. When you're at the end, just pop the main account into the game by pressing 'A' on that controller and bam, achievement.
I did this accidentally while going for my 4-player co-op achievement. During the cinematic at the end of level 2-1, I jumped the other 3 accounts in and the two that weren't Guests unlocked this achievement. So, there ya go. =D