AddEventHandler("sale", "OnBasketAdd", "AddPresentToBasket");
function AddPresentToBasket($ID, $arFields)
{
    CModule::IncludeModule("iblock");
    CModule::IncludeModule("sale");

    $arIBlockFilter = array(
        "IBLOCK_ID" => 27, // pair_products
        "ACTIVE" => "Y",
    );

    $dbBasketItems = CSaleBasket::GetList(
        array("NAME" => "ASC", "ID" => "ASC"),
        array("FUSER_ID" => CSaleBasket::GetBasketUserID(), "ORDER_ID" => "NULL")
    );

    $arCurrentBasket = array();

    while($arBasketItem = $dbBasketItems->Fetch())
    {
        $arCurrentBasket[] = $arBasketItem['PRODUCT_ID'];
    }

    $dbIblockElements = CIBlockElement::GetList(array("ID" => "ASC"), $arIBlockFilter, false, false, array("ID", "IBLOCK_ID"));
    while($obIblockElement = $dbIblockElements->GetNextElement())
    {
        $arIblockElementProps = $obIblockElement->GetProperties();
        if(!empty($arIblockElementProps['PARENT_PRODUCTS']['VALUE']) && !empty($arIblockElementProps['ADDITIONAL_PRODUCT']['VALUE']))
        {
            foreach ($arIblockElementProps['PARENT_PRODUCTS']['VALUE'] as $arParentProduct)
            {
                if($arParentProduct == $arFields['PRODUCT_ID'] && !in_array($arIblockElementProps['ADDITIONAL_PRODUCT']['VALUE'], $arCurrentBasket))
                {
                    Add2BasketByProductID($arIblockElementProps['ADDITIONAL_PRODUCT']['VALUE'], 1);
                }
            }
        }
    }
}