i think that <meta> tags with the 'refines' attribute were only introduced in epub3, and that the only dublin:core tags that allowed attributes (specifically the 'role' attribute) were the 'creator' and 'contributor' tags (i.e., <dc:creator opf:role="aut"... <dc:contributor opf:role="dsr"... ).
so basically, in epub2, you can't use <meta refines... for a subtitle, and you can't have a second <dc:title/> element with the attribute opf:role="subtitle".
it seems like part of the reason the idpf is introducing the meta tags to refine roles in epub3 is actually that people/publishers were complaining that they couldn't include subtitles in the metadata.
https://code.google.com/p/epub-revis...d_Subtitles_(A)