Two questions about Amazon account sharing vs separate accounts for the Kindle

[Tidalcloud]

First - between husband and wife, which would recommend? I guess this question dovetails with whether you would share the same account for Amazon purchases. Pros include access to Prime Instant for both users, single purchase history, subscription management, access to all digital purchases for both users etc. Cons include recommendations not especially accurate to either user as it's shared. Wishlist is a combined list etc.
Second, and what has led me to ask this question. My wife has made Kindle book purchases with my login and with her own. She now owns Kindle books tied to both accounts. She received an iPad for her birthday and I installed the Kindle app to it. I am now in the midst of the question as to how to provide the app with access to all of her Kindle books and how to suggest she handle this moving forward.

Thanks all.