What does it mean to say that Kant has a twofold ontology? If indeed he does have a twofold ontology. Is that like Descartes' res extensa and res cogitans?
Saying that Kant had a twofold ontology (like DesCartes) is saying that Kant believed the world was made of two types of objects (two ontologically separate categories).
DesCartes ontology is twofold because of his mind-body dualism: DesCartes held that the mind and the body were made of fundamentally different substances that can never be reduced to one another.
Kant's ontology is similarly twofold, although his approach is more from the point of view of knowledge and epistemology: Kant asserted that there was a Noumenon, a world of ideas and truths (Ding an sich - things-in-themselves) that exists beyond what can be known purely from the senses (Phenomenon - things as we see them/feel them). Again although he was looking at it from an epistemic point of view, ontologically this can be seen as a form of dualism because the things-in-themselves are different from the-things-as-we-perceive-them.