Right I've been trying for this one periodically for around 2 years and it's finally worked, probably due to a patch. But on the safe side, use these parameters:
You can use any controllers.
Have all the controllers signed into a profile - can use guests, silver or gold accounts.
Have everyone use the Red/Blue/Green/Orange Knight. No Greys or Kings etc.
Have them all fall on one screen, on the same level, at the same time. Lava World with the eruptions is a good example.
It doesn't have to be perfect, but be sure to aim to press Y on the heart in the little timing game.
Any questions please message me. This one was a real pain.