Can be easily obtained in solo: simply use a second controler with a dummy profil and play with two controlers/profils. Merge/unmerge the two characters (
) is still possible, so you can play most of the time as in solo.
I have not finished all the levels for now, so chapter 3 (little frogs) was the shortest and easier to get it for me.
You must have the two profils connected to the game before entering the chapter; if the second player join the game after the chapter is entered the trophy won't unlock.