Ever use a
Tapwave Zodiac?
It's got a 480x320 pixel screen,
all of which is touch sensitive. The unit can also be rotated 90 degrees so that the you have a 320x480 vertically-oriented touch screen.
Keep in mind that you're capable of doing split-screen on this device. A developer could hypothetically port a DS game over to Zodiac, position the unit in portrait (vertical) orientation, do a top-and-bottom split screen, and either of the Zodiac's logical "screens" would have two definite and unarguable advantages over the DS's hardware restrictions, no matter how you try to spin it:
1. Either Zodiac screen could be touch sensitive, as opposed to only the lower screen on the DS.
2. Either Zodiac screen is higher resolution than either DS screen.
There's also the fact that you could do a custom split, such as a 320x320 area on top and a 160x320 area on the bottom. You could also merge the screens back into one large 320x480 screen area, with no physical borders or breaks. You could also rotate the device and get back to 480x320. You could do all of this on one unit, and even within the same game if the game designer wanted to. On the contrary, you can't do any of this
at all on the DS because of the way the hardware is designed.
Before someone decides to ask the obvious follow-up question about screen protection, here's the answer: You use the included screen flap, which can fold behind the unit when not in use (or you can remove it altogether if you want).
Size comparison with original Game Boy Advance:
So, now exactly
what is the advantage of having two tiny screens as opposed to a single, high resolution, completely touch sensitive screen?