Reviewing for my own benefit since I'm trying to figure this out myself: The store you're allowed to use is based on your address. Amazon isn't actually all that strict about it (unlike e.g. iTunes), but due to different distributors having paid for the rights to certain books only in certain territories, you're not actually supposed to use multiple stores. Unless you have multiple residences or something, I guess. There's certainly no official support from Amazon for a multi-store scenario because that'd make all their publishers mad.
If you're a Canadian with a US Amazon account buying the Japanese translation of Hunger Games...which publisher would get paid?
An Amazon.com account is almost certainly not going to have official access to Amazon.co.jp books, so just switching your residence in your account page won't make the Japanese content appear. They're ultimately two different stores.
I'm wondering myself about how to wrangle content from multiple stores. My current idea is just to keep one Kindle tied to one store, and side-load all the de-DRM'ed books that I've bought from the other store as personal docs.
I could just side-load them directly from calibre, but that kinda defeats the purpose of the Kindle's ability to sync books and annotations between all your devices.
They'll certainly be DRM-locked, but we'll have to wait and see if the existing tools will work as-is or not.