You have to get 3 stars in the 6 levels of the world 2. You don't need to do the Kevin's levels.
Here is a playlist containing videos of all levels unlocked in 4 stars duo mode
. They will show you the technique to use, but you just have to make 3 stars for this achievement
See also below the 6 levels separately. Good game.
Level 2-1 :
Level 2-2 :
Level 2-3 :
Level 2-4 :
Level 2-5 :
Level 2-6 :