Wait...
If you read some of Riku's bios on websites, they say Riku was the original key bearer, but he fell into Darkness instead because he thought he could save Kairi that way. So on Destiny Islands when he and Sora were caught in the Darkness (when Sora got the Keyblade) it chose Sora too. Once Riku came back to the Light, he still had to power of Darkness, so his keyblade came into existence, in short, Twilight, or the road to Dawn...
Hope that explains
please no flamming