Reassign variables in XSLT

My problem is that I'm trying to map some values which are in different nodes, I have to mache them, the thing is that I can not reassign the variables so that I can do the right checks. I would like to know some way to save the value of traversing, machear and assign another value to x variable to be able to return to traverse and machear until it ends.

    <xsl:for-each select="//List1">
  <xsl:variable name="id1" select="id1"/>
  <xsl:for-each select="//List2">

    <xsl:variable name="id2" select="id2"/>
    <xsl:choose>
      <xsl:when test="$id1 = $id2">
        "iguales"
      </xsl:when>
      <xsl:otherwise>
        "no hay coincidencias"
      </xsl:otherwise>
    </xsl:choose>
  </xsl:for-each>
</xsl:for-each>