If the two Kindles are registered to the same account, they will have access to the same books. For example, I have 5 Kindle devices and 3 'kindle apps' registered to my account. I can read any book on my account on any of the 8, though, generally, I can only have it downloaded to, at most, 6 devices simultaneously.
For books borrowed via Prime Lending, they are on the account and available to any registered device. You can even re-download it if you have not yet returned it via MYK (Manage Your Kindle).
We do have a Fire Talk board and you might get more complete information from the Fire users over there.
