[Mitchell06]

Amazon has tons and tons of books available for Kindle, and have chosen the standard AZW format as the one their reader uses, meaning readers can not read Kindle books on other E-readers freely or convert them to something else directly because the Kindle DRM protection.
epub is a widely accepted format, so here demonstrates how to enjoy Kindle books on Nook,Kobo,iPad freely that you legally purchased in Amazon.
Best solution demonstrations for kindle books reading on other devices!
(http://www.epubsoft....zw-to-epub.html)

[Mitchell06]

nook works fine with its official site pulished books, but this is not equal to the kindle readers. Kindle has its exclusive book storefront the same, amazon provides books that suit kindle reading perfectly.
I am afraid you cannot share the books you bought from barns&noble shop directly with other one. but things alawys got the corresponding solution.just make use of a software to convert nook epub book format to kindle supported azw one, then it can be possible for you two to share books all the time.
epubsoft ebook converter will be helpful for you.