Super Mario & Sonic 2
From Sonic Retro
Super Mario & Sonic 2, also known as Super Mario & Sonik 2 and Super Mario Sonik 2 is a pirate hack of Chip & Dale for the Nintendo Entertainment System. Unfortunately, only one level is playable before the game locks up, though you do get to chose between four pairs of characters who behave identically (Mario & Sonic, Chip & Dale, Mario & Mario and Sonic & Sonic). Mario's sprites are edited from Super Mario Bros. 3, but Sonic's are completely custom and odd looking.
The one playable level takes place in a kitchen, where you can throw boxes at mice. Other levels from the original game likely exist, but can only be accessed through hacking.