Dissidia sephiroth isn't really a canon interpretation, especially since, if taken logistically, all the characters in Dissidia are basically perfect Manikin copies of their respective characters, since there is no logical way any of them just suddenly disappeared from their game, and it's more likely their memories and wills were drawn from their worlds to create the warriors used by Cosmos and Chaos, and later with Materia and Spiritus.
It's more likely that Advent Children Sephiroth dissapated into the lifestream and the Geostigma he infected the planet with allowed him to take control of a fraction of the Whispers (making them purple), and manipulated them to both change the destiny of his original incarnation, and lead them to their destruction at the hands of the party so he could be reformed.
Although that last part is kinda conjecture, but a ton of the visual symbolism kinda leads to the idea that he absorbed the whispers to use their power to recreate himself, then fought the party to further encourage them to try to defeat him for real. Wether he actually reformed and there are two sephiroths, and the one messing with you nearly the whole game was actually AC Sephiroth exerting his will through the Whispers, or he just clued his past self in, I'm not sure.
I think there is enough evidence to support the two sephiroth idea though, and I welcome discussion on that idea.
Two sephiroths? Or just one with fortelling from the future and power over some of the whispers?
