Fans have been doing remarkable things with Mario games on the Wii despite the presumably long list of limitations that make the task a challenging one. That creative push from the homebrew community continues with this new hack/mod which adds splitscreen cooperative play to Super Mario Galaxy 2 via game patcher Riivolution.
As you'll undoubtedly notice from the demonstration, it's not quite there yet. For starters, both Mario and Luigi are being controlled by a single person, since no one else was around to help. Elements of the HUD and the camera also need some work, as YouTube uploader MrBean35000vr describes at length.
Even still, the project is looking promising already and I can't help but be impressed. Though there isn't a solid release window yet, don't expect it to go public for a while.