I'd like to share something I've been thinking about for a bit:
If the title symbolifies the telling of two people's stories over the course of 358 days, and Riku is one of those people, wouldn't that work out a bit awkwardly?
We know Roxas's 358 days start when he joins the Organization, which is right after his birth - near the very end of KHI. And it ends, of course, when he is defeated by Riku.
So how would they work out Riku's 358 days without having a rehash of CoM's R/R? Would it start right after that, and go into the time Roxas was held in the virtual TT?
I'm all for a Riku mode in Days, though. It really would help flesh out the story. Not to mention it would be rather odd to suddenly swap to Riku in-between Roxas's missions.