Returned Xion in DLC Explanation

[KeybladeMasterJr]

As stated by Nomura, "I think it’s something that people could guess, but we plan to explain it in downloadable content."

I still don't get it, I played it over and over again and still think she's trapped in there and since we can now duel wield as Sora, this just confirms my suspicions, it makes no sense if he could since all the hearts trapped in his are released?....now it brings up another question...how come he didn't duel wield in kh 1 when he had Kairis heart in his?

 

[okhi12]

Kairi didn't have a keyblade at that point, and probably didn't know what a keyblade was even though she was a potential wielder already. She had forgotten about having met Aqua...
And the dual wield is not related to the number of hearts anymore, otherwise Sora would tetra-wield in KH2, Terra would dual wield and Terra-Xehanort would tri-wield.
Sora is the key that connects everyone (whatever that means) and probably retains that ability through the power of friendship aka his connections to others.

 

[KeybladeMasterJr]

Technically Xehanort in Kh 3 can with his flying keyblades lol but ya makes sense

 

[AR829038]

They did explain how she came back. And let me clear, there is definitely not another heart still trapped within Sora. There were only ever three: Ventus, Roxas, and Xion. When Sora fought with Xion at the Keyblade Graveyard, her memories within him flowed into her, hence the head-grabbing and wailing she was doing. And even though we don't see it, it is also at this point that her heart jumps back into her body as well.

 

[SweetYetSalty]

To add on, her portrait is shown in Roxas's heart at one point but once the memories return to her, her portrait is gone. You can take that as a clue that her heart returned to her as she is a being made up of memories. She was a squatter in Roxas all along lol.