second

$collection = Mage::getModel('catalog/product')
->addAttributeToFilter('category_id', array("in"=>array('26')))
->addAttributeToFilter('category_id', array("in"=>array('63')));

foreach($collection as $_product){
    echo $_product->getName()."<br/>";
};

// There has been an error processing your request
Exception printing is disabled by default for security reasons.