How to add a Custom Meta Box for more than one Post Type?

Whenever in doubt about a WordPress function, consult the Codex.
http://codex.wordpress.org/Function_Reference/add_meta_box

There, you’ll see that you need to add one meta box to each post type.

add_action( 'add_meta_boxes', 'myplugin_add_custom_box' );

function myplugin_add_custom_box() {
    add_meta_box( 
        'myplugin_sectionid',
        __( 'My Post Section Title', 'myplugin_textdomain' ),
        'myplugin_inner_custom_box',
        'post' 
    );
    add_meta_box(
        'myplugin_sectionid',
        __( 'My Post Section Title', 'myplugin_textdomain' ), 
        'myplugin_inner_custom_box',
        'page'
    );
}