Amazon has put up the product pages for the 4K Ultra HD versions of Batman: Assault on Arkham and Batman: The Killing Joke, revealing the release date. According to the listings, both physical releases will be coming out on the 11th of September in the North American region. If you are interested, you can preorder the movies with the following links from the online retailer:
You can also find the 4K Ultra HD cover for both Batman animated films with the images located just below this paragraph.
In addition to the 4K Ultra HD disc, the two product also includes a Blu-ray disc and digital copy of the same films.