HoT... how could you trade with or battle against multiple files on the same cartridge? That would require you to... like... cut it in half and have a working half in one DS and a working half of the cartridge in another DS so they both can be played at the same time....
Anyway, the reason there's only one file, originally, was because there wasn't enough memory to have two. (Though they always talk about the lack of memory as an excuse for everything, and I wonder how much memory the Missing No. and other glitches take, what with their like quantillion possibilities.) I really used to think that there should be multiple files, but I have to say, playing older games on emulator, I have the opportunity to have like 10 files with the save states, and I never have effectively used more than 1. In my experience, I have no use for multiple files, I get attached to one team and keep playing them and don't want to move on, but I guess in the case of sibling rivalry you could help it out with multiple files on a cartridge, but then you'd also have to work out a who-gets-to-play-when schedule, if both (or more) kids become addicted. =/