You can have more than one xsl:sort element.

They provide addition sorts for when all the previous sorts are equal.

Example:

<xsl:template match="employees">
  <employees>
    <xsl:apply-templates select="employee">
      <xsl:sort select="name/last"/>
      <xsl:sort select="name/first"/>
    </xsl:apply-templates>
  </employees>
</xsl:template>