What is a Split King? A Split-King consists of two Twin XL mattresses laid side-by-side. Check out which of our mattresses are offered in a Split-King, HERE Related articles Does my Purple® Mattress have to be split in order for the Ascent Adjustable Base to work correctly? Do you offer a Split-King adjustable base where each side can be set to their own preferences? What mattress foundation should I use? What kind of mattress protector do I need? Does Purple sell mattress toppers?