var xml1 = XDocument.Load("file1.xml");
var xml2 = XDocument.Load("file2.xml");

//Combine and remove duplicates
var combinedUnique = xml1.Descendants("AllNodes")
                          .Union(xml2.Descendants("AllNodes"));

//Combine and keep duplicates
var combinedWithDups = xml1.Descendants("AllNodes")
                           .Concat(xml2.Descendants("AllNodes"));