Phantom Hourglass had a competitive online mode in which one player, as Link, would run through a dungeon trying to grab Force Gems and bring them to his scoring area while the other player took control of some of the game's monolithic Phantoms, drawing paths for them to follow in an attempt to intercept Link. It was actually quite excellent in my opinion and extened the life of the game for several weeks.
I am not sure if Spirit Tracks followed suit since I have only played about half of the main game. It might though.
But I take it you don't mean a seperate mode but rather having two people playing through the main game together. While I don't think I would like having someone else accompany me on the entire adventure (I prefer to go at my own pace while exploring the world and having someone else might interrupt my natural play-style) there are two ideas which come to mind.
The first was raised by
this thread from last month.
Having dungeons which revolve around two playable characters was an interesting idea in The Wind Waker and I wouldn't be averse to seeing it return. But instead of making us take control of the companion via a song or magic, it would be interesting if you could simply swap between two characters Resident Evil 0 style.
However, why not allow another player to join you as you enter the dungeons and they take direct control of the companion character? That way you can still complete the dungeon if you play alone but you have the option there to work as a team to finish dungeons. This also allows people like me to play through the adventure at my own pace, since in dungeons you tend to always be more focused on your goal so there is less wandering around and staring at scenery.
The other idea is less interactive but still a nice touch. It is in fact the only thing that I actually liked about Fable 2 (for those who don't know, I hate Fable 2 with a furious passion). In Fable 2, people on your Friends List who were playing the game at the same time would appear in your game as an orb. This orb would float around as your friend moved about their game, showing you exactly where they were and letting you know what they were doing. Now, in Fable 2 you could use these orbs to trade items and even join each other's games (in which you ended up playing not as your Hero but a stunted character and the screen was tethered to both of you, hindering exploration and combat) and something similar would be interesting for Zelda. Especially with the Miiverse, having a visual representation of your friends in-game would not impact on your own personal adventure but still help foster a greater sense of community and you could even include the ability to send messages by interacting with the orb.
Imagine seeing a friend's orb in a dungeon, apparently stuck on some fiendish puzzle. You approach the orb, interact with it, type out a little message offering them a hint and then, in their game, the Postman runs up. "You have a letter from X. My work is done. Onward to mail!" Your friend opens up your letter, sees your hint and solves the puzzle. Would that not be charming?
