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'
);
}