In all Unity games I saw so far, every object is either 2D (with RigidBody2D and Collider2D components), or 3D (with RigidBody and Collider components). This makes sense, but I wonder if there is any use to an object that has both (for example, both RigidBody2D and RigidBody, or both Collider2D and Collider)?
Are the worlds of 2D and 3D completely parallel and unrelated, or can they be connected somehow?