The way I figured, after mulling it over when it was new, was that it was supposed to be "Three Hundred Fifty Eight 'Half' Days" like what VoidGear said. It would've made sense if you consider that you play through 358 days(the year Sora was asleep, minus the week Roxas spends in virtual TT) as half a person, so it's like only half living them.
That made sense to me, and is kinda meaningful, but hearing all this that it's officially Three-Five-Eight Over Two and that the "over two" means its about two people just makes the title seem stupid. It's not very clear which two people the 358 is "over" and shouldn't be in the title as a defined numerical value if no one can see what it's talking about.
Overall, my opinion on 358/2: It's a complex title, maybe forgivably so if it holds metaphorical depth like "358 days lived by half a person." Less forgivable if Over 2 is some needless complication added to make the title more interesting looking. Ah, I accidentally defined Xion.