To St. Severus, an "ousia" was a generality, or an idea, not something concrete that exists. So to be made of two generalities to St. Severus makes it seem like neither nature had any real existence, only the thought of what that existence is made of. What he does call them may surprise you. Since the word "hypostasis" means existence, or concrete thing, St. Severus talked about a "self-subsistent" hypostasis and a "non-self-subsistent" hypostasis. In other words, the divinity is a self-subsistent hypostasis, because it existed by itself before the incarnation. The humanity's existence (hypostasis) depends on the divinity's self-subsistent existence (hypostasis), and therefore, while the humanity has a real existence (hypostasis) on its own by thought, both the humanity and divinity make one composite existence together (mia hypostasis). The word "nature" or "physis" St. Severus mentioned can either mean ousia or hypostasis. In this case, he would mention he used the term "physis" meaning "hypostasis," i.e. a concrete existence.
The problem with this, and I know this is something that people think, is that the word "hypostasis" seemed to have been defined by Chalcedon as synonymous to "prosopon." Therefore, even in that word, we differ in meaning. If St. Severus was to read St. John of Damascus's Christology on the "enhypostasis" theory, he would have thought St. John was a Eutychian, because the "enhypostasis" theory to St. Severus would mean that the existence of humanity is the divinity, rather than depends on the divinity. To St. Severus, a "self-subsistent hypostasis" would be a prosopon with concrete existence, while a "non-self-subsistent hypostasis" is a concrete existence without a prosopon (or self). If you read Fr. VC Samuel's evaluation and comparison of the theologies of St. Severus and St. John, Fr. VC Samuel asks the question "Does the Damascene acknowledge that the humanity exists or not?" using Severian thought. Otherwise, in all other things, both St. Severus and St. John agree fully in essential Christological thought.