I think, i found the solution.
I have modified the cart class (classes/cart.php) - getPackageShippingCost function.
I add this code before the shipping cost is converting to float:
$product_array = $this->getProducts();
$pcount = 0; //quantity of products in cart
$moreThanOneQuantity = FALSE;
foreach($product_array as $product_item) {
$pcount++;
if($product_item['quantity']>1) $moreThanOneQuantity = TRUE;
}
if($shipping_cost>0 && ($pcount>1 || $moreThanOneQuantity )) $shipping_cost=$shipping_cost/2;
Hope it will help someone.