The two words are very close. I can find examples were one is much more used than the other (like disséminer with indices), but it’s hard to find some kind of “rule of thumb” as to which one to choose.

To me, éparpiller is more associated with a messy, involuntary scatter, while disséminer can be more voluntary, like carefully hiding clues here and there, for example.

Éparpiller is also slightly negative, like it’s not the goal, the “things” were supposed to stay grouped. Whereas disséminer fits more when it’s a “good thing” that the things are spread everywhere. It’s in accordance with the voluntary vs involuntary thing.

It’s not a strong “rule” though, take it with a grain of salt. But it fits in most examples.