[@robin-w Moderator 7 years, 10 months ago]

you need to alter ‘post’ to topic and reply eg (untested)

function adding_custom_meta_boxes( $post_type, $post ) {
    add_meta_box( 
        'my-meta-box',
        __( 'My Meta Box' ),
        'render_my_meta_box',
        'topic',
        'normal',
        'default'
    );
add_meta_box( 
        'my-meta-box2',
        __( 'My Meta Box 2' ),
        'render_my_meta_box',
        'reply',
        'normal',
        'default'
    );

}
add_action( 'add_meta_boxes', 'adding_custom_meta_boxes', 10, 2 );