This got me thinking about the other games I'd played and how they handled the two screens, because in many cases I simply had not noticed. But in a few cases, the mechanism stands out as both innovative and a complement or enhancement to the game play.
There are, so far (there's always room for innovation), essentially three or four generic mechanics for handling the two screens that I have seen:
- Separate worlds, separate screens
- Ignore the gap
- The invisible game space: the DMZ
- The invisible game space: playing in the dark
Separate Screens, Separate Worlds
Separate screens is also very common for platformers and "educational" titles (such as Brain Training). The advantage of this method is that the gap becomes a non issue. The disadvantage is that if you don't have much additional content, the second screen is essentially wasted. This is very noticeable in some of the early titles such as Ridge Racer and Rayman, where the bottom screen is primarily a very bad replacement for an analog stick.
Ignore the Gap
In this mode the game ignores the physical gap and acts as if the two screens are two adjoining segments of a seamless view. This avoids any issues of what happens "in the gap", but does create a bit of a discontinuity as objects "jump" across the physical gap between the screens.
As a side note, I can't think of any games that are designed this way. It is possible and even likely that some game has created a partitioned game field ignoring the gap. But in most cases where a game uses both screens for the same "environment", they use one of the following modes to handle the gap.
The Invisible Game Space: The DMZ
In this mode the two screens form a single playing field and the gap between them is treated as part of the field -- an invisible space. However, the game ensures that the player either never enters that space or is "safe" while passing through.
Note that both the player and enemies may pass through this space, but not together since that would risk a collision or attack in the invisible space. Examples of this are Yoshi Touch n Go where the enemies pass through the gap but Yoshi doesn't. Except in the first scene where baby Mario is falling and only after the enemies have cleared the area (as Baby Mario makes the final fall to be caught be Yoshi).
The Invisible Game Space: Playing in the Dark
One of the best examples I have seen of this technique is in Bomberman where "tunnels" lead through the gap from the top screen to the bottom and the player (or enemies) can use the tunnels to hide bombs or to trap opponents with blasts from one screen to the next.