Magento (CE1.7) Catalog Category Layer Filter: Filtering for the attribute gender. Adding a second value to this attribute for the filter query. In this example we add the unisex value to the query when someone searches for a product which gender attribute is lady or gentlemen.

Attribute2.php

<?php
/**
 * XML rewrite:
 *
 * <config><global><models>
     <catalog_resource>
         <rewrite>
             <layer_filter_attribute>Namespace_Module_Model_Resource_Layer_Filter_Attribute</layer_filter_attribute>
         </rewrite>
     </catalog_resource>
 * </models></global></config>
 *
 * */
class Namespace_Module_Model_Resource_Layer_Filter_Attribute extends Mage_Catalog_Model_Resource_Layer_Filter_Attribute
{

    /**
     * Apply attribute filter to product collection
     *
     * @param Mage_Catalog_Model_Layer_Filter_Attribute $filter
     * @param int                                       $value
     *
     * @return Mage_Catalog_Model_Resource_Layer_Filter_Attribute
     */
    public function applyFilterToCollection($filter, $value)
    {

        $collection = $filter->getLayer()->getProductCollection();
        $attribute  = $filter->getAttributeModel();
        $connection = $this->_getReadAdapter();
        $tableAlias = $attribute->getAttributeCode() . '_idx';

        $conditions = array(
            "{$tableAlias}.entity_id = e.entity_id",
            $connection->quoteInto("{$tableAlias}.attribute_id = ?", $attribute->getAttributeId()),
            $connection->quoteInto("{$tableAlias}.store_id = ?", $collection->getStoreId()),
        );

        $unisexValue = Mage::helper('namespace_module')->getUnisexValue();

        if ($attribute->getAttributeCode() === 'gender' && (int)$value !== $unisexValue) {
            $conditions[] = $connection->quoteInto("{$tableAlias}.value in (?)", array((int)$value, $unisexValue));
        } else {
            $conditions[] = $connection->quoteInto("{$tableAlias}.value = ?", $value);
        }

        $collection->getSelect()->join(
            array($tableAlias => $this->getMainTable()),
            implode(' AND ', $conditions),
            array()
        );

        return $this;
    }

}