that doesn't mean all worlds will be one, just that kingdom hearts will create a new world.It's in the MX Reports. As well as from KH1. Kairi's Grandma's story was essentially a fairy-tale about the Keyblade Wars. At the end, the once united realm was broken into two. If someone were to open Kingdom Hearts, they could make a new world that was essentially how it original was (one realm, all the worlds as one). Etc.
As of yet though, no one has actually opened KH. (Probably how the series as a whole will end)
as for realms, i'm sure it use to be one until keyblade wars