I happened to receive an old NES with Super Mario Bros. My kids pointed out that there's a two-player mode where both Mario and Luigi play at the same time. Does anybody know how to get it going?
FWIW, the cartridge says "European Version":
"Select" should highlight 2-player and then "Start" should begin the game. I believe that 2 players at the same time is a 2018 ROM hack, so it won't work on your cartridge.