Which kindle devices, exactly, support two page layout spreads and which do not?
All Kindle devices that support fixed layout ebooks (so, the Paperwhite and the various versions of the Fire) should support two page layouts. It's important to note, though, that in the Kindle Publishing Guidelines, the only place that two-page spreads are mentioned is in Section 5: Creating Fixed-Layout Graphic Novels/Manga/Comics. In Section 4: Creating Fixed-Layout Children's Books the only way that page spreads are demonstrated is by making a single html page that looks like two print pages. In other words, if what you want is a book that will display a single page in portrait and two facing pages in landscape, you need to make sure it's a comic rather than a children's book.