Why is it called 'Kingdom Hearts 358 2 Days'?

Answer 1

Kingdom Hearts 358/2 Days is titled the way it is, most likely because of the amount of days that Roxas existed. He endured 358 days as an actual member and the 2 other days are described as '358/2' because although they are part of his existence, they take place after he leaves and 'turns' on the organization.

Answer 2

Because eh...

Roxas spent 358 days in the organization,

When roxas wakes up from the simulated twilight town he thought the whole organization was a dream. Hence 358/2

To sum it all up he spent 358 days in his mind in over 2 days

Amazing isn't it

Answer 3

358 is the amount of days in the organization, the 2 being his days in twilight town