add_action( 'woocommerce_cart_calculate_fees', 'discount_on_2nd_cart_item', 10, 1 );
function discount_on_2nd_cart_item( $cart ) {

    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    // baslangic
    $count = 0;
    $percentage = 50; // 50 % indirim orani

    // sepetteki her ogenin islenmesi
    foreach ( $cart->get_cart() as $cart_item ) {
        $count++;
        if( 2 == $count){ // yalnizca ikinci urun
            $price = $cart_item['data']->get_price(); // urun fiyati
            $discount = $price * $percentage / 100; // hesaplama
            $second_item = true;
            break; // donguyu durdur
        }
    }
    if( isset($discount) && $discount > 0 )
        $cart->add_fee( __("2. Ürüne %50 İndirim Kampanyası", 'woocommerce'), -$discount );
}