I would use the LINQ .Concat method. The problem is that the XmlNodeList is IEnumerable as opposed to IEnumerable< XmlNode > . So you should call .Cast< XmlNode > on XmlNodeLists .
For example:
var List = nodesoriginal.Cast<XmlNode>().Concat<XmlNode>(nodesupdate.Cast<XmlNode>());
Tim armstrong
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