As many people probably already know, most touch screens cannot register multiple simultaneous touches. Such screens do exist, but they're not common, and the Nintendo DS's screen definitely isn't one of them.
Given this limitation, you would have no way of forming chords on an on-screen key panel as depicted above. But just for fun, I was thinking about how IIDX could actually be accomplished on a DS....
There actually was a Game Boy Color version (more than one, in fact) of DDR. Obviously, DDR's controls are simpler with only 4 arrows that naturally map to a D-pad. However, anyone who's played DDR on a controller knows that hitting LR or UD jumps on a D-pad is generally impossible. As such, they mapped A and B to left and up so that you could form jumps. They also had a physical adapter that you could snap onto your Game Boy that presented the familiar DDR controls as pictured here:
Underneath were little plastic tabs that would actuate the buttons underneath. Pressing the L or D button would actually press L or D on the D-pad and pressing U or R would press A or B.
I suppose you could do something similar for IIDX by building an adapter that had 7 keys on it that would physically actuate the real A,B,X,Y,and D-pad underneath. Then you could leave a portion of the touch screen visible for scratching by just swiping your finger up and down on it--something the touch screen
would be able to recognize.
The only problem with this is that you'd only have 6 keys because you could only really use two of the D-pad directions for the same reasons as DDR (you can't press opposite directions at the same time). So they'd either have to be sure no charts had chords that would do that or somehow get the adapter to actuate the shoulder buttons. I suppose that would be possible, but I think it would be a mechanically hairy and rather hulking adapter at that point.
Though I think IIDX would lose a lot on the DS platform since there's not much storage space on the carts. There probably wouldn't be much music, it would get downsampled quite a bit, and you can pretty much forget about the video aspect of IIDX.
I know I've put too much thought into what was clearly just a fun little joke, but it was kind of a fun little exercise. And if it were real, I might buy it just for the sheer novelty factor in spite of everything else.
