I suspect that what they want is akin to the Send/Receive function (which I believe may have dropped off the future feature list) that makes it so that one person can send it and another receive to continue working on it, then send it back. Our Enabling Writers grantee in Nigeria apparently ran into this problem, with wanting editors to actually work on the books in Bloom on different computers than the authors. So they needed to get the Bloom books onto another computer without ending up with two versions of the book being worked on separately.
The only way I am aware of to do this right now would be for one person to zip a book’s folder and send it to the second person who unzips it into a collection on their computer for the same language. They just have to agree who is currently in control of the book and can make changes. Bloom doesn’t have any built-in tools for sharing individual books in editable (not shell book) form as far as I know.
With regard to adapting a book for all languages in the project, I believe they would do that by creating a Bloom Pack with the book in it that could be installed on other computers and used in collections for the other languages in the project. So, Bloom does have tools for that kind of sharing of books across languages.