they don't, though. canonically they have the same voice. hayden voices both kairi and xion in the games she appears.
Yeah, but so far SENA has done such a weird job of going back and forth with them (where so far Hayden has mostly voiced Kairi, and Alyson, Xion), that it almost seems like they do/can. Though to be fair, I know a lot of this is probably because of Hayden's busy schedule, though.
That idea drives me up the wall, but to each their own. It'll be interesting to hear from the cast again.
I can see why it would bother you (and it used to me, too, since they're not voiced like that in the Japanese version), but I just like it this way now, because we all know Xion's going to become her own person in the end anyway, so why not go the extra mile and give her her own voice? It serves to help differentiate her from Kairi as well, which is nice, as a lot of people (rightfully, tbh) complain about the number of carbon copies of people in this series.
And I... I just can't imagine anyone but Hayden as Kairi most of the time, and while I could
maybe handle Hayden voicing Xion (since I don't have my nostalgia goggles on as tightly with her), I feel like that would also come off as really strange, perhaps.
So really--for me, at least--giving them different voices is truly the best way to go. Especially since it would give both Hayden and Alyson the chance to end out this saga they've both spent quite a time acting in.