Well it took me a while, but my solution was to reverse it. Use the custom post the primary admin menu option and then you can add a standard admin page as a submenu by doing the following:
$labels = array(
'name' => _x( 'Manage Lists', 'mmd_list' ),
'singular_name' => _x( 'Manage List', 'mmd_lists' ),
'add_new' => _x( 'New List', 'mmd_list' ),
'add_new_item' => __( 'Add New List' ),
'edit_item' => __( 'Edit List' ),
'new_item' => __( 'New List' ),
'all_items' => __( 'Manage Lists' ),
'view_item' => __( 'View List' ),
'search_items' => __( 'Search List' ),
'not_found' => __( 'No Listing found' ),
'not_found_in_trash' => __( 'No Listings found in the Trash' ),
'parent_item_colon' => '',
'menu_name' => 'Lists'
);
$args = array(
'labels' => $labels,
'description' => 'This post type holds all posts for your directory items.',
'public' => true,
'menu_position' => 10,
'show_ui' => true,
'supports' => array( 'title' ),
'has_archive' => true,
'menu_icon' => 'dashicons-media-spreadsheet',
);
register_post_type( 'mmd_list', $args );
add_submenu_page('edit.php?post_type=mmd_list', // Parent Slug from
add_menu_page
'Settings', // Title of page
'Settings', // Menu title
'manage_options', // Minimum capability to view the menu.
'mmd_list_options_slug', // Unqiue Slug Name
'mmd_maplist_DrawAdminPage' ); // A callback function used
to display page content.
The key is to swap out the parent slug for the custom post type:
edit.php?post_type=”Your Post Type”