Dear me,

Really interesting as I was asking me the same one.

You have simply to add the following xml fragment into Advanced Setting > XML Extensions > New > Choose a name:

  <extension target="org.nuxeo.ecm.platform.actions.ActionService"
    point="actions">
    <action id="view_comments_myproject" link="/incl/tabs/document_comments.xhtml"
      enabled="true" order="80" label="action.view.comments">
      <category>VIEW_ACTION_LIST</category>
    <filter id="commentable_michelin2">
      <rule grant="true">
        <type>MyDocType1</type>
        <type>MyDocType2</type>
        <condition>currentDocument.isProxy()</condition>
      </rule>
    </filter>
    </action>
  </extension>

Where MyDocType1 and MyDocType2 are the 2 doc types where you want to have the comments on their proxies.