You're assuming that Xion's Keyblade is real, and that Keyblade can bounce owners just like that. Oh, and that Xion didn't take her Keyblade with her to kingdom come.
Also, you'd have to ask then where did Riku's Duel Blade come from. Since if you use up Xion's Keyblade to explain Roxas's Duel Wielding, you'd have to explain Riku's in another way (not that I support that as an explanation to either).
And that's before we get to the point that Sora and Roxas both wielding at the same time to begin with is a complex matter that already put's Xion's Kingdom Key (which no doubt came from SoRo in some manner to begin with) as a redundant factor in the whole mess. They wouldn't need her for another Keyblade.