Not from one account to another, just to different Kindles registered to the same account.
If the books were bought on different accounts and the Kindles (or apps) are on these different accounts, I don't believe they can be transferred without de-registering the Kindle from the original account and re-registering it to the new account. I have that problem: I have a gift Kindle registered to the gifter's account, with access to over 300 books. I bought a few books on my own account and wanted to add the, and there was no way to do that unless I de-registered from the other account and registered to mine.
I think the only thing you can do is "loan" each other books. I haven't done it, and it has to be a book that allows loaning (not all do), and I think there is a limit to how many/how often (publisher determines). Go into your Kindle library on Amazon.com. At the end of each book's listing is a drop down called "Actions." If the book is lendable, you can click on loan, and it will bring up a menu asking for the information on who to deliver it to. It will also tell you the details of the loan. For instance, my first book listed is Dances With Wolves. It says that the publisher has set the lending terms: the book can be loaned once for a period of 14 days.