While you may be right about RSS, Atom 1.0 does specify a format for listing multiple authors. It looks something like this:
<author>
<name>Jane Doe</name>
</author>
<author>
<name>John Doe</name>
</author>
Whether or not all feed readers handle that is a different matter. Both Safari 5 and Google Reader, for instance, display only the last author in the list. This may be because the interaction model and UI for these readers were designed with RSS in mind and so they neglect to support the more advanced features of Atom.