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//for AND
    $collection = Mage::getModel('sales/order')->getCollection()
    ->addAttributeToSelect('*')
    ->addFieldToFilter('my_field1', 'my_value1')
    ->addFieldToFilter('my_field2', 'my_value2');

    echo $collection->getSelect()->__toString();

//for OR - please note 'attribute' is the key name and must remain the same, only replace //the value (my_field1, my_field2) with your attribute name


    $collection = Mage::getModel('sales/order')->getCollection()
        ->addAttributeToSelect('*')
        ->addFieldToFilter(
            array(
                array('attribute'=>'my_field1','eq'=>'my_value1'),
                array('attribute'=>'my_field2', 'eq'=>'my_value2')
            )
        );


link : http://stackoverflow.com/questions/3826474/magento-addfieldtofilter-two-fields-match-as-or-not-and