The Big Boss contains the familiar special sauce, pickles, lettuce, onions, and a sesame seed bun, but comes with crispy white meat chicken filets rather than beef patties, and cheddar instead of American cheese.
If the concept sounds familiar, McDonald's actually offers a Chicken Big Mac in certain parts of the world, although the Chicken Big Mac (or just "Chicken Mac" depending on location) is made with pre-formed McChicken patties rather than filets.
While the press release claims the sandwich is a "world-first," KFC has also been offering a markedly similar sandwich over in Pakistan since last year, but using the name "KFC Double Decker." Perhaps it doesn't contain "special sauce."