merge tabs in woocommerce

the content from additional information is shown above the description
content

Because the following code calls wc_display_product_attributes() which displays the default content for the additional information tab:

do_action( 'woocommerce_product_additional_information', $product );

To fix it, you can turn on output buffering, run the do_action() and then get the output using ob_get_clean():

function customizing_woocommerce_description( $content ) {

    // get attributes
    global $product;

    // Only for single product pages (woocommerce)
    if ( is_product() ) {

        // The custom content
        ob_start();
        do_action( 'woocommerce_product_additional_information', $product );
        $custom_content = ob_get_clean();

        // Inserting the custom content at the end
        $content .= $custom_content;
    }

    return $content;
}