Speaking in the latest edition of "Iwata Asks," Nintendo engineer Masato Kuwahara revealed that the initial prototype for DSi had two DS slots. Nintendo originally planned to let gamers store up to 2 DS titles in their handheld at one time, but as this ultimately increased the build size, the idea was scrapped at the last minute.
Company president Iwata noted that the decision did end up pushing back the release date of DSi, but in the end, Nintendo felt it made the right choice. But did they? I personally think so. While I can see the appeal of having dual slots, I'd much prefer a slimmer, less bulky handheld.
Iwata Asks [Nintendo]