This is for completing the bonus level called "All for One, One for all" and it's the hardest level in the game because you have to control 3 characters at once. It is unlocked when you collect all 60 Ankh ☥ collectibles.
I got very stuck on this one, so I made a video to share as soon as I figured it out. Timing is key at some parts otherwise the guy on the right will step on a skull. At 0:10 in the video, I teleport up just before the plants projectile is about to hit me. This is perfect timing to get the guy on the right off the skull.
Anyway, it's hard to explain in text, but hopefully the video is enough. Let me know if you have any questions.