"Tandem" publishing is simply the happy marriage between physical & digital publishing – as it involves a Book (in any o its forms) . . . and an accompanying Website.
Whether for family or public consumption, the Book carries the essence of the story itself – while the Website carries the real detail – the underpinnings, back stories, references, maps, & timelines . . . not to mention updates, author perspectives and observations, music, sound, and animation – things a book cannot deliver.
He or she who reads the Book will want to be "informed" by the more interactive Website. And she or he who is even mildly engaged by the Website – will have to have the book to make the "story" come true.
While the Book monetizes the project (if it's not just meant for family), the actual "Work" is comprised of BOTH the Book and the Website. With so many advantages accruing from the marriage: A bigger, deeper, far more expressive (& digitally enhanced) platform for the Author, a bigger, deeper, more fulfilling story for the Reader, and a totally natural and organic, "medium-is-the-message" propellant for the Marketing department!
These examples are from our "Milestones" department.