The 'Magic finder methods' works only with single properties, as stated in docs you need to extend your repository (pseudo code)
public function findByCategoryAndAuthor(Tx_MyNews_Domain_Model_Category $category, Tx_MyNews_Domain_Model_Author $author){
$query = $this->createQuery();
$query->matching(
$query->logicalAnd(
$query->equals('category', $category),
$query->equals('author', $author)
)
);
return $query->execute();
}
and use:
$newsRepository->findByCategoryAndAuthor($category, $author);