Sorry i dont have this recorded, but i was messing around playing this co-op with my daughter earlier and found two things.

1: right as she entered a pipe, i grabbed a warp star. this made us switch positions, and left her floating in midair spinning, unable to move. after a couple seconds, she randomly warped into the wall on the left edge of the screen, and every time jump was pressed, her mage shifted upwards one tile. Im not sure if or where this would be useful, but yeah.

2: She was a ghost, i was alive, she was near the very bottom of the screen, and i was near the top. she grabbed a warp star while dead and partially offscreen, which caused me to warp to where she was, instantly killing me because i was at the bottom of the screen. this resulted in me coming back as a ghost, leaving us both as ghosts. The screen continued scrolling endlessly at the speed it was moving when i died until the end of the stage, where we were softlocked since we were both ghosts and couldnt enter the door, and there were no chests/boxes to try to get a powerup to revive with from where this weird dual-ghost anomaly started until the end of the stage.

Recording of #1:

thanks for grabbing a vid of that! Hopefully it ends up being useful somewhere in the co-op route, its such a neat little glitch

Semi-related: for funsies, I checked if anything particular happened when all players die at the same time. Nope!