See http://wordpress.stackexchange.com/a/191963/35541

MetaboxStealer.php

<?php

/**
 * @author  Giuseppe Mazzapica <[email protected]>
 * @license http://opensource.org/licenses/MIT MIT
 */
class MetaboxStealer
{
    /**
     * @var bool
     */
    private static $stealing;

    /**
     * @var string
     */
    private $post_type;

    /**
     * @var array
     */
    private $boxes = [];

    /**
     * When the request contain special variable, this function will make it
     * return a serialized version of $wp_meta_boxes array and die.
     */
    public static function init()
    {
        add_filter('post_updated_messages', function ($messages) {
            if (MetaboxStealer::stealing()) {
                ob_start();

                return [];
            }

            return $messages;
        });

        add_action('do_meta_boxes', function () {

            if (MetaboxStealer::stealing()) {
                ob_end_clean();
                global $wp_meta_boxes;
                echo serialize($wp_meta_boxes);
                die();
            }
        });
    }

    /**
     * Checks that the request contain a special variable that will make request
     * return a serialized version of $wp_meta_boxes array and die.
     *
     * @return bool
     */
    public static function stealing()
    {
        if (is_null(self::$stealing)) {
            $screen = function_exists('get_current_screen') ? get_current_screen() : null;
            $stealing = filter_input(INPUT_GET, 'stealing-boxes', FILTER_SANITIZE_STRING);
            self::$stealing =
                $screen instanceof \WP_Screen
                && $stealing
                && wp_verify_nonce($stealing, $screen->post_type);
        }

        return self::$stealing;
    }

    /**
     * @param string $post_type Current post type
     */
    public function __construct($post_type)
    {
        $this->post_type = $post_type;
    }

    /**
     * Send a HTTP request to post edit page of a given CPT setting a special
     * variable that will make that page return serialized $wp_meta_boxes array.
     * After that, so obtained boxes are merged with the boxes for current post type.
     *
     * @param string $cpt CPT to steal metaboxes from
     */
    public function steal($cpt)
    {
        $vars = [
            'post_type'      => $cpt,
            'stealing-boxes' => wp_create_nonce($cpt),
        ];
        $url = add_query_arg($vars, admin_url('/post-new.php'));
        $cookies = [];
        foreach ($_COOKIE as $name => $value) {
            if ('PHPSESSID' !== strtoupper($name)) {
                $cookies[] = new \WP_Http_Cookie([
                    'name'  => $name,
                    'value' => $value,
                ]);
            }
        }
        $response = wp_remote_get($url, ['cookies' => $cookies]);
        if (! is_wp_error($response)) {
            $body = wp_remote_retrieve_body($response);
            if (is_serialized($body)) {
                $boxes = unserialize($body);
                $this->boxes = isset($boxes[$cpt]) ? $boxes[$cpt] : [];
                empty($this->boxes) or $this->merge();
            }
        }
    }

    /**
     * Merge meta boxes from current request with boxes obtained from HTTP request to another CPT.
     */
    private function merge()
    {
        global $wp_meta_boxes;
        isset($wp_meta_boxes[$this->post_type]) or $wp_meta_boxes[$this->post_type] = [];
        foreach ($this->boxes as $context => $priorities) {
            foreach ($priorities as $priority => $boxes) {
                if (! isset($wp_meta_boxes[$this->post_type][$context])) {
                    $wp_meta_boxes[$this->post_type][$context] = [];
                }
                if (! isset($wp_meta_boxes[$this->post_type][$context][$priority])) {
                    $wp_meta_boxes[$this->post_type][$context][$priority] = [];
                }
                $wp_meta_boxes[$this->post_type][$context][$priority] = array_merge(
                    $wp_meta_boxes[$this->post_type][$context][$priority],
                    $boxes
                );
            }
        }
    }
}

How To Use

// init the class
add_action('admin_init', ['MetaboxStealer', 'init']);

// use the class to merge boxes for current CPT with boxes for another CPT
add_action('edit_form_after_editor', function ($post) {
    $stealer_cpt = 'post';
    $steal_from = 'page';
    if ($post->post_type === $stealer_cpt) {
        $stealer = new MetaboxStealer($post_type, $post);
        $stealer->steal($steal_from);
    }
});