The thing with Sora/Roxas was more so that it was 358 days that they were separated.
When Roxas was put into the fake Twilight Town, he was essentially in Sora's Heart/Memories, meaning they werent truly separate.
That's how the case for that goes.
If you ask me, Xion/Roxas is the "obvious" two people, that aren't it.
No it definitely is Xion & Roxas that share the 358 days. It's proven throughout the game by devoting the entire story to those two characters, and more specifically by how the keyblade's power fluctuates in between the two. It's pretty obvious that those are the two that it really applies to.
The reason Sora & Roxas are the false duo that people would assume to be correct is because before the game came out, people didn't know who Xion was or what kind of a connection she had to Sora and Roxas. Therefore, people assumed that because Roxas was Sora's Nobody, that was the only connection established.