<root>
  <pair/>
  <p> 1 </p>
  <p> 2 </p>
  <separator/>
  ...
  <pair/>
  <foo>bar</foo>
  <bar>baz</bar>
  <separator/>
  ...
</root>

<root>
  <feature>
    <p> 1 </p>
    <p> 2 </p>
  </feature>
  <feature>
    <foo>bar</foo>
    <bar>baz</bar>
  </feature>
</root>

<xsl:for-each select="pair">
   <feature>
      <xsl:variable name="sep" select="count(following-sibling::separator)"/>
      <xsl:copy-of select="following-sibling::*[not (name() = "separator"") and 
              count(following-sibling::separator) = $sep]"/>
   </feature>
</xsl:for-each>

<xsl:copy-of select="following-sibling::separator[1]/preceding-sibling::*"/>