I love how people say there's nothing to support Riku Duel Wielding when Nomura all but said in Another Report (that OmniChaos quoted but people didn't read before posting) that what Riku gave to Kairi was the Way to Dawn. Him refering to it as "Kairi's Keyblade" is also easily nulled by the fact that the Oblivion was initially called Riku's Keyblade. Does it really make it Riku's? No, it's just to show the connection.
Kairi held an unnamed Keyblade, thus the best way to refer to that Keyblade is "Kairi's Keyblade" - which Riku handed to her.
It's never seen again afterwards.
Xemnas ignored her when hailing the Heroes of the Keyblade.
I'm not ruling out her being what I liked to call a User - she can Use the Keyblade, namely - she can hold it without it running away screaming like it did with Leon and arguably with Jack (it was obvious Sora didn't want the Keyblade to stay with him, there's nothing saying it couldn't stay with Jack).
I highly doubt she's a Wielder though, being - she has her own Keyblade that she can summon at will.
As for the claim many like to bring -
"Xion gave Riku her Keyblade and Riku gave it to Kairi!"
For now, I believe the Keyblade Xion uses is Roxas's, until we'll get either a scan showing them both wielding a KK at the same time (as even in Duel Wielding, one cannot use the same Kechain, it would seem), or the game that will tell us otherwise.
As for giving it to Riku - it would mean that for the period of time between Riku receiving it and handing it to Kairi - it was his own Keyblade, for better or worse, much like the KK was Sora's until Riku snatched it back in KH1's HB.
Which would mean that for the duration of time between him acquiring the WtD (which would not have been Xion's Keyblade - it sprouted from the Soul Eater) and handing Kairi the Keyblade, he was effectively chosen by two Keyblades, something I'm having a hard time digesting.
But then we have Nomura saying that what Riku gave Kairi was the WtD, so :\ No.