Just to clarify for everyone else. Regarding the quote that you posted, spoken by Nomura:
The original two that were expected to "share the 358 days" were Sora & Roxas. However, we all know that what Nomura was trying to put across was that the time period was shared by Roxas & Xion.
It always seemed to me that the creaters were always trying to push Roxas and Xion together (rather forcefully if you ask me), so it make sense that they were create the title name with the two in mind. I'm sure that before Days came out, we all had different expectations of what was to happen in it so that's why Nomura would say that by the time we beat the game, we would believe it to be a different 'two' people.
In any case, my thought process about the title meaning before I read Nomura's quote was that the title could also be understood to possibly mean, Kingdom Hearts 358 Days [and then Kingdom Hearts] 2. I don't know Japanese or anything, it just seems to happen that sometimes words are translated over in one way, that perhaps meant something different in Japanese. Just some possible spectulation that the title could have a double meaning with the 'two people' and also possibly 'Kingdom Hearts II'.