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Buy one copy. Amazon explicitly allows you to put your Kindle purchases on up to 6 different devices (pending publisher restrictions).
If you and your wife would like to keep separate accounts for housekeeping purposes (not overriding each others' sync/highlights, different tastes in books and purchase budgets), you can just do the de-register/re-register swapping trick to get a properly DRM-ed copy of the book to put on the "other" Kindle, which is again something that Amazon allows.
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