It's two Mario objects, upon initialising the second object the flag that usually dictates whether it is Mario or Luigi that is loading is set to the opposite state, resulting in one Mario, one Luigi (and both characters have the physics that they're supposed to, i.e. Luigi comes with his low traction, etc.). As for the split screen, that was made from scratch by halving the render size of the game world, and on alternating frames, rendering the game world from both characters' perspectives
P.S. He spelled initializing wrong.