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Sora and Xemnas...hmm I can't feel any personal grudges between the both of them compared to Riku VS Xehanort's Heartless.
*Sora has Ventus's heart within him, and Xemnas was obsessed with finding Ven and the Chamber of Waking throughout 358/2 Days. Beyond that, there is even a hint in the final battle between them where Xemnas says "Can you spare a heart?" and he may very well have been trying to take Ven's heart since Xemnas himself doesn't want a heart.
I find that very interesting. I heard this same Theory somewhere before.
Sora sees Riku as Xemnas and Riku sees Sora as Ansem so when they fight each other, they won't actually be fighting each other. This will allow Yen Sid to see how strong Sora and Riku really are and test their limitations.