While it's possible to change the default field on which products are sorted, it is not possible 1) to define anything other than ASC order and 2) one cannot have a multi-key sort. To actually do this requires some code.
// Default getLoadedProductCollection() in /template/catalog/product/list.phtml
$_productCollection=$this->getLoadedProductCollection();
// Reset default sorting preference and add our own
$_productCollection->clear();
$_productCollection->getSelect()->reset(Zend_Db_Select::ORDER);
$_productCollection->addAttributeToSort('price', 'DESC');
$_productCollection->addAttributeToSort('name', 'ASC');
Supposedly it's possible to define this in a layout.xml override but I could never get it to work.