I don't know anything about Kobo, but Kindles are limited to one Kindle device per account, so you can't share an account across two Kindles, and you can't share books across two accounts. It's one of the only big frustrations I've found since both my wife and I got our Kindles. There isn't any restriction on the Kindle app running on other devices however, so you can access the same account through a Kindle and through the Kindle app on an Android tablet (for example), just not through two Kindles.
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