Think about it. The bottom screen would be the main world. When you go inside a house, or a cave, that action appears on the top screen. And through WiFi, the bottom screen of the main overworld is always shared between players, but the action on the top screen is private to those in that specific area. In other words, the top screen would play the role of the GBA in Four Swords Adventures. The difference is, Four swords for the DS would use WiFi, so it wouldn't be limited to the living room, and you wouldn't need extra hardware.