using System;

using System.Linq;

using System.Xml;                   

using System.Xml.Linq;

using System.Collections; 

using System.Collections.Generic;

using System.Data;

using System.Data.DataSetExtensions;

using System.Data.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Xml.Serialization; 

using System.IO;

using System.Globalization;

using System.Diagnostics;

using static System.Console;            

​

public class Program

{

    public static void Main()

    {

        string input =@"<RootNode>

<SubNode>

​

<NodeA id=""1"" />

<NodeB id=""2"" />

<NodeC id=""3"" />

​

</SubNode>

<LonelyNode/>

</RootNode>";

        string input2 = @"<RootNode>

        <SubNode>

​

<NodeD id=""1"" />

<NodeE id=""2"" />

<NodeF id=""3"" />

​

</SubNode>

<LonelyNode/>

</RootNode>";

        var doc = XDocument.Parse(input);

        var doc2 = XDocument.Parse(input2);

        doc.Root.Element("SubNode").Add(doc2.Root.Element("SubNode").Elements());

        doc.ToString().Dump();

    }

}

​

​