Hi,
   You can use a temporary node and copy the contents of first into
this temp node. Then remove the first node and append the temp node at
last of the xmlDocument.
   Hope i got your problem.

Try
  XmlNode sibling = xmlNode1.NextSibling;
  if (sibling != null) {
    xmlNode2.ParentNode.InsertBefore(xmlNode1, xmlNode2);
    sibling.ParentNode.InsertBefore(xmlNode2, sibling);
  }
  else {
    XmlNode parent = xmlNode1.ParentNode;
    xmlNode2.ParentNode.InsertBefore(xmlNode1, xmlNode2);
    parent.AppendChild(xmlNode2);
  }