From what I've heard, the Soul Eater was the "stem" as I'll call it, for the Way to Dawn. In CoM, Riku learned to accept the Darkness and the light. Maybe he was still learning how to do this which is why he still had the Soul Eater at the end of CoM and still had it when he was working with DiZ to watch over Roxas.
Possibly after Roxas merged with Sora, Riku was still learning how to deal with Darkness and light and while this was happening, Way to the Dawn was formed because he was to be the original Keyblade master, sorta like what Hades' ideas were. That's my guess.
P.S. For some reason, they called Soul Eater a Keyblade in the KH1 strategy guide, but the Dark Keyblade a sword, but that could be a translation error.
Possibly after Roxas merged with Sora, Riku was still learning how to deal with Darkness and light and while this was happening, Way to the Dawn was formed because he was to be the original Keyblade master, sorta like what Hades' ideas were. That's my guess.
P.S. For some reason, they called Soul Eater a Keyblade in the KH1 strategy guide, but the Dark Keyblade a sword, but that could be a translation error.