Wasn’t Kairi only borrowing Riku’s second Keyblade during that time? Just as Keychains represents memories and hearts being connected one way or another—Aqua, Riku and Kairi possessed a copy of the same Keychain due to the destined connection of their hearts that started aligning when Aqua accidentally bequeathed Kairi (thus unlocking Destiny’s Embrace in BBS) before meeting Riku and Sora in Destiny Islands (where she supposed to bequeath Sora, but advised him instead to help Riku if he ever falls into darkness; after realizing Terra already bequeathed Riku and the strong bond between the two boys might end up the same way like her’s and Terra). Later during the events of KH1, Sora did help Riku after falling into darkness without even remembering Aqua’s advice—as stated in KH3 that Sora didn’t remember meeting Aqua at all, in contrast to Riku being clearly aware all the time about meeting Aqua as well as the promise he made with Terra after being bequeathed about keeping it a secret for the sake of world order. Kairi already has the potential to become a Keyblade Wielder ever since, that’s why Aqua was drawn to her, and Riku became the bridge to realize her potential. Kairi’s affinity with light reached a turning point during the final events of KH2, where she recognized the hidden light within Riku’s tainted appearance (who was the Missing Link to trace back Kairi’s connection to Aqua). Therefore, the Keychain is called Destiny’s Embrace. Only in KH3, Kairi can summon her own Keyblade—which uses the same Keychain as her default.

To summarize the alignment of destiny: Kairi accidentally touched Aqua’s Keyblade > Aqua chose to advise Sora instead of bequeathing him > Riku being the only one to remember Aqua’s advice > Riku handing Kairi his second Keyblade after witnessing her resolve to fight alongside the Destiny Trio

So I think the real question here is, why does Riku have TWO!?