I don't know if someone has done something similar before, but here's a mod that let's you play smb3 as two players simultaneously. If you choose 1 player game, player 1 always controls Mario, and player 2 controls Betty (an Orange Cheep Cheep). If you choose 2 player game, the non-main player controls Betty, i.e. if Luigi is playing, then Mario controls Betty and vice versa.
For now, Betty is mainly a liability for the main player, as she can't really do anything, she just has to run along. I suspect that making her interact with enemies would be too resource intensive on the poor NES, but I've been thinking of making her be in charge of collecting coins and maybe even bumping "?" blocks for Mario/Luigi. Or maybe she should be able to shoot fire balls?
There are a lot of bugs to fix, but it's pretty much playable as it is now, but only on maybe half of the levels, for various reasons, mainly because she has not learned how to swim (ironically) or jump on platforms.