Ok. Here's part of an interview with Shigeru Miyamoto and Sartoru Iwata, two owners of Nintendo:
Iwata: Ever since Mario Bros., you've had your heart set on making a multiplayer Mario game. You've tried each time, but it's never quite come together… Even with Mario 64, it started with Mario and Luigi running around together, didn't it?
Miyamoto: That's right. The screen was split and they went into the castle separately. When they meet in the corridor, I was incredibly happy! (laughs) Then there was also the mode where the camera is fixed and we see Mario running away, steadily getting smaller and smaller.
Iwata: Yes, that's right.
Miyamoto: That was a remnant of an experiment we did where Mario and Luigi would run away from each other but you could still see them both. But we were unable to pull it off…
