It really makes sense to have Riku's story in Days. 358/2 likely means two different stories during the span of 358 days. This is probably why Nomura says that you have to beat Days to understand the title. Also, I can't see them showing so many Riku scenes from what we've seen so far even though it's supposed to be the story of Roxas. Likely, some of the scenes we've seen are from Riku's mode already.