One custom post type cannot be the child of another. hierarchical means: Each post can have children of the same post type, like pages.
If you need relationships between different post types use the Posts 2 Posts plugin.
Another option would be a custom hierarchical taxonomy for two post types. You could use it to assign the first post type to the parent terms and the second to the child terms of that taxonomy. That’s probably as messy as it sounds. 🙂