Paper Mario Multiplayer 1.2 is a fan-made modification of Paper Mario 64, enabling multiplayer functionality. The version number suggests updates and improvements to the multiplayer aspect. The precise features of the multiplayer mode are unknown without more information, but it likely allows multiple players to experience the game simultaneously in some capacity.  It has no official publisher and is a fan project.

Paper Mario Multiplayer 1.2

Paper Mario 3D 64