I want to publish my book both in two languages simultaneously. Is there a special procedure available that can be helpful in keeping both versions connected? Or do I just have to publish both versions in isolation?

Yes, we do have a feature that lets you "connect" a book and its translation into another language.

You can see this feature by going to the Book Tools menu for your book in Leanpub, clicking "Book Info" and then clicking "Book Details."

On this page, you will see a "Translated From" dropdown menu that will show all of your books. If you create a new book, go to this same page in the Book Tools menu for the new book, and select the book it is translated from. Then, click the blue "Update Book" button.