You could try
Code:
<dc:creator opf:role="aut">Charles Sheffield</dc:creator>
<dc:creator opf:role="aut">Jerry Pournelle</dc:creator>
or better
Code:
<dc:creator opf:role="aut" opf:file-as="Sheffield, Charles">Charles Sheffield</dc:creator>
<dc:creator opf:role="aut" opf:file-as="Pournelle, Jerry">Jerry Pournelle</dc:creator>