I believe there are hacked Secret of Evermore versions out there where a 2nd player can control the dog.

Also, Twisted Tales of Spike McFang has a code where the 2nd player can control Rudy (google it). One of the coolest codes around, although, I don't know why they didn't just make that an option from the get-go (they could have advertised the game as a 2-player action RPG and it probably would have sold more). Note that the RPG-ness of Spike McFang is extremely light however. It's really more of an action adventure.