As we can read on Super Mario Wiki, Super Mario 64 2 is a cancelled sequel to Mario 64 for the Nintendo 64, that was planned for the 64 DD expansion by Nintendo but never finished. The game was talked many times in interviews with Miyamoto but it seems that only a early prototype (with a multiplayer mode starring Luigi) was created before they decided to stop the development and just start a new Mario for the GameCube.
It is possible that some concepts of this game were later included in Super Mario 64 DS, Super Mario Sunshine or Super Mario Galaxy. Nintendo never shown any screens from the Mario 64 2 prototype, but we can still hope that they could release some images in one of the “Iwata Talks” articles in the future.
Some old interviews with Miyamoto that talks about Mario 64 2:
We’re in the middle of preparing Mario 64-2 for release on the 64DD. I’d like to take advantage of the 64DD’s ability to store information. As of now, Luigi’s also a full part of the game, but we haven’t started thinking about 2-player gameplay with Mario and Luigi yet. We’ll tackle that once we’ve got the system ironed out—we’ve figured out the processing power issues, so we could do it if we tried. How many Luigi fans do you suppose there are? (Editing department replies: “Quite a lot.”) If Luigi’s really that popular, maybe I’ll made a green box for Mario 64-2. (laughs)
Thanks to Parker for the contribution!