A split adjustable base is usually in reference to a split adjustable base king mattress. Instead of having one adjustable base that controls your entire mattress, you have two adjustable beds – one for you and one for your significant other. This also requires two mattresses instead of one. For example, two Twin XL mattresses are the same size as a King, so for a split adjustable base, you could have two Twin XL mattresses instead of the King. The good thing is that the two mattresses usually cost the same as the king, so you don’t have to pay extra so you can elevate your head if your partner doesn’t.