I've got to join in and say that Double Dungeons is one of the best 2-player experiences I've had on the Turbo. Zooming through the dungeons and building up levels and equipment to take down the tougher non-random monsters that guard the way to the more advanced parts of each dungeon is a great time. It's fine as a single-player game, but having a friend along makes it into a race to see who gets the best weapons/finds the boss first. I also like how every time you encounter the other player, the text box always displays the other guy's name with an exclamation point.
Plus, like many other first person dungeon crawls, the lack of an auto-map combined with the huge size of the later dungeons means this game can seriously improve one's spatial awareness with extended play.
The other game I had to mention is Last Alert. Sure the voice overs are goofy but there is some solid run 'n gun action to be had.