A WordPress plugin that tests Admin Page Framework to see how to add a custom post type sub-menu item to another custom post type.

APFTest_CPTGroup_Admin.php

<?php

class APFTest_CPTGroup_Admin extends AdminPageFramework {

	public function setUp() {
		$this->setRootMenuPageBySlug(　'edit.php?post_type=cpt_a' );
		$this->addSubMenuItems(
			array(
				'title'        => 'Settings',
				'page_slug'    => 'apf_test_multiple_cpt_group',
                　　　　　　　　　'order'        => 30,
			)
		);
	}

}

APFTest_CPTGroup_PostType_A.php

<?php
class APFTest_CPTGroup_PostType_A extends AdminPageFramework_PostType {

    public function setUp() {
        $this->setArguments(
            array(
                'labels'                => array(
                    'name'                  => 'CPT A',
                    'menu_name'             => 'CPT A',
                ),
                'show_ui'               => true,
                'menu_position'         => 200,
                'show_submenu_add_new'  => false,   // an admin page will be placed instead
                'submenu_order_manage'  => 10,
            )
        );
    }

}

APFTest_CPTGroup_PostType_B.php

<?php
class APFTest_CPTGroup_PostType_B extends AdminPageFramework_PostType {

    public function setUp() {

        $this->setArguments(
            array(
                'labels'                => array(
                    'name'                  => 'CPT B',
                    'menu_name'             => 'CPT B',
                ),
                'show_ui'               => true,
                'show_in_menu'          => 'edit.php?post_type=cpt_a',
                'submenu_order_manage'  => 20,
            )
        );

    }

}

test-admin-page-framework-cpt-group.php

<?php
/**
 * Plugin Name:    Admin Page Framework - Test CPT Group
 * Plugin URI:     http://admin-page-framework.michaeluno.jp/
 * Description:    Demonstrates how to add a custom post type sub-menu item to another custom post type.
 * Author:         Michael Uno
 * Author URI:     http://en.michaeluno.jp/
 * Version:        0.0.1
 */

class APFTest_CPTGroup {

    public function __construct() {
        add_action( 'plugins_loaded', array( $this, 'load' ) );
    }
    public function load() {

        if ( ! class_exists( 'AdminPageFramework' ) ) {
            return;
        }

        include( dirname( __FILE__ ) . '/APFTest_CPTGroup_Admin.php' );
        include( dirname( __FILE__ ) . '/APFTest_CPTGroup_PostType_A.php' );
        include( dirname( __FILE__ ) . '/APFTest_CPTGroup_PostType_B.php' );
        new APFTest_CPTGroup_PostType_A( 'cpt_a', null, __FILE__ );
        new APFTest_CPTGroup_PostType_B( 'cpt_b', null, __FILE__ );
        new APFTest_CPTGroup_Admin;

    }

}
new APFTest_CPTGroup;