using System;

using System.Linq;

using System.Xml;

using System.Xml.Linq;

public class Program

{

    public static void Main()

    {

        var doc1XML = @"<doc1><first></first></doc1>";

        var doc2XML = @"<doc2><second>2</second></doc2>";

        var doc1 = XElement.Parse(doc1XML);

        var doc2 = XElement.Parse(doc2XML);

        var firstElement = doc1.Descendants("first").FirstOrDefault();

        var secondElement = doc2.Descendants("second").FirstOrDefault();

        firstElement.Add(secondElement);

        //Console.WriteLine(firstElement.ToString());

        Console.WriteLine(doc1.ToString());

    }

}