Yes, you can bring that former condition back.
To explain: A Kindle is Registered to an Amazon.com account. You buy e-books in the Amazon.com account, and if in the Kindle edition, they appear on the Kindle that is Registered to that account.
Thus, if you Register both Kindle devices to the same Amazon.com account, then both Kindles can buy e-books, and an e-book bought on one Kindle (either one) appears on the other Kindle too, on both Kindles really.
When you Register to a given Amazon.com account, any e-books stored on the Kindle that were bought from a different account, tend to become either inaccessible or erased. You cannot have a Kindle registered to more than one account at once, you see.
So if one of your two Kindles is registered to the other Kindle's account, that Kindle will lose its current e-books, regrettably.
Accounts may be merged again, so that both account's purchases may be obtained on the unified account. Contact Amazon.com Customer Support to request such an account merger, after deciding which Amazon.com account you will want on both Kindles, if you wish, at 1(###) ###-####.
If this answers your question, please rate my answer using the five-star interface on this page:
A five-star rating would be appreciated!
If you need more info, or have any comments, use Reply To Expert to respond.