Unfortunately, if you are using Propel 1.4 (Symfony 1.4), it seems you will have to switch to raw SQL there ...
There is no answer on the forum (old) Symfony: http://oldforum.symfony-project.org/index.php/m/90447/
However, since Propel 1.5 should work with the new syntax (Doctrine style). It is not described in detail in the Propel document, but when I check this, it works (gives ORDER BY author.name ASC, author.id DESC).
$authors = AuthorQuery::create()
->orderByName()
->orderById('desc')
->find();
Perhaps consider upgrading to Propel 1.5.