Think of this bed as two twin XL mattresses pushed together. This type of king-size mattress is usually designed to work with an adjustable base and to cater to each sleeper's needs. One sleeper can sit up, while the other rests in a horizontal position.
These mattresses are good for reducing motion transfer, as each sleeper has their own mattress. This means, you likely won't be able to feel your partner moving around. The downside is that these mattresses tend to cost more and require specific sheets.