Hackers do what Nintendon't: Super Mario Galaxy 2 Splitscreen Co-Op Mod

It's really impressive to see the kind of stuff people can do when they get their hands dirty in some of the game's code and turn it into something else.  Project M, DayZ, even fan translations for games like Mother 3, are all cool mods. Soon you'll have the ability to play 2 player splitscreen co-op in Super Mario Galaxy 2 where each player plays as one of the brothers instead of a cursor on the screen. It's apparently still early in its development with obvious hitches and no release date, but it shows a lot of promise.

The modder behind the project goes into detail in the comments about how he gets it to work:

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.