Notes (and I apologize if anyone posted notes like this before):
Both A and X can be used to advance text boxes, independent of which set of characters are having the text boxes.
It's faster to have Luigi open boxes that you need to go past because he's taller.
If you're in a position where you can open two Attack Piece Boxes at the same time (the Trash Pit is a good example), whatever brother hits the box first opens their box. The other box does not open. It may be possible to hit both boxes on the same frame, though I don't know what brother gets priority in that case. Probably Mario.
Because the doors in the Trash Pit are locked until you gather the attack pieces, there's no skipping getting Green Shell.
Green Shell feels easier for me to do in realtime than at frame-level.
Jumping likes this frame best: