Show a custom post title in another custom post type in Meta box

You need to get all the titles of CPT (Custom Post Type) A, and loop through them in the second CPT in a metabox of CPT B.
Example :

$my_titles = array(
    'post_status' => 'publish',
    'posts_per_page' => -1,
    'order' => 'ASC',
    'post_type' => 'my_custom_post_type' // get the type of your CPT
);

Now in your second custom post type i.e B, inside the meta box loop through them like

    <select name="" id="">
        <?php foreach ($my_titles as $key => $title): ?>
            <option value=""><?php echo $title; ?></option>
        <?php endforeach ?>
    </select>