function fasting_and_fitness( $user_id, $user_tags ) {

    if ( wpf_has_tag( 'Fasting System', $user_id ) && wpf_has_tag( 'Fitness System', $user_id ) && ! wpf_has_tag( 'Fasting And Fitness System', $user_id ) ) {

        $tag_id = wp_fusion()->user->get_tag_id( 'Fasting And Fitness System' );

        wp_fusion()->user->apply_tags( array( $tag_id ), $user_id );

    }

}

add_action( 'wpf_tags_modified', 'fasting_and_fitness', 10, 2 );