add_filter( 'woocommerce_product_tabs', 'vagelis_dev_merge_tabs', 99 );
function vagelis_dev_merge_tabs( $tabs ) {
    $tabs['description']['callback'] = function() {
        global $product;
        the_content();
        do_action( 'woocommerce_product_additional_information', $product );
    };
    unset( $tabs['additional_information'] );

    return $tabs;
}