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Support › MB Custom Post Type › Custom Post Type menu within another Custom Post Type? › Reply To: Custom Post Type menu within another Custom Post Type?
It appears this is possible only through code (not within the meta box builder) by specifying show_in_menu => 'edit.php?post_type=[your_custom_post_type_slug]'
Reference: https://codex.wordpress.org/Function_Reference/register_post_type