C# Online Compiler | .NET Fiddle

using System;
using System.IO;
using System.Text;
using System.Collections.Generic;
using System.Xml.Serialization;
					
public class Program
{
	public static void Main()
	{
		List<InvoicePair> data = new List<InvoicePair>
            {
                new InvoicePair(new EInvoice{Version="1.03"},new TradeInvoice{Id=1,Value=100}),
                new InvoicePair(new EInvoice{Version="1.03"},new TradeInvoice{Id=2,Value=200}),
                new InvoicePair(new EInvoice{Version="1.03"},new TradeInvoice{Id=3,Value=300})
            };

		MemoryStream stream = new MemoryStream();
		Root.Serialize(data, stream);
		
		Console.WriteLine(Encoding.UTF8.GetString(stream.ToArray()));
		
		stream.Seek(0, SeekOrigin.Begin);
		
		var deserialized = Root.Deserialize(stream);
	}
}


public class InvoicePair
{
	public InvoicePair(EInvoice eInvoice, TradeInvoice tradeInvoice)
	{
		TradeInvoice = tradeInvoice;
		EInvoiceInfo = eInvoice;
	}
	
	public EInvoice EInvoiceInfo { get; set; }
	
	public TradeInvoice TradeInvoice { get; set; }
}

[XmlRoot("RootXML")]
public class Root
{
	public Root()
	{
		RootBodies = new List<Invoice>();
	}
	
	[XmlElement(Type = typeof(EInvoice), ElementName = "e-Invoice")]
	[XmlElement(Type = typeof(TradeInvoice), ElementName = "TradeInvoice")]
	public List<Invoice> RootBodies { get; set; }
	
	public static void Serialize(IEnumerable<InvoicePair> invoices, Stream stream)
	{
		Root root = new Root();
		foreach (var invoice in invoices)
		{
			root.RootBodies.Add(invoice.EInvoiceInfo);
			root.RootBodies.Add(invoice.TradeInvoice);
		}
		
		XmlSerializer serializer = new XmlSerializer(typeof(Root));
		serializer.Serialize(stream, root);
	}
	
	public static IEnumerable<InvoicePair> Deserialize(Stream stream)
	{
		XmlSerializer serializer = new XmlSerializer(typeof(Root));
		Root root = serializer.Deserialize(stream) as Root;
		
		for (int i = 0; i < root.RootBodies.Count; i += 2)
		{
			yield return new InvoicePair((EInvoice) root.RootBodies[i], (TradeInvoice) root.RootBodies[i+1]);
		}
	}
}

[XmlInclude(typeof(EInvoice))]
[XmlInclude(typeof(TradeInvoice))]
public abstract class Invoice
{
	
}

[XmlRoot("e-Invoice")]
public class EInvoice : Invoice
{
	[XmlElement("Version")]
	public string Version { get; set; }
}

[XmlRoot("TradeInvoice")]
public class TradeInvoice : Invoice
{
	[XmlElement("Id")]
	public int Id { get; set; }
	[XmlElement("Value")]
	public int Value { get; set; }
}

            yield return new InvoicePair((EInvoice) root.RootBodies[i], (TradeInvoice) root.RootBodies[i+1]);
 
using System;
using System.IO;
using System.Text;
using System.Collections.Generic;
using System.Xml.Serialization;
                    
public class Program
{
    public static void Main()
    {
        List<InvoicePair> data = new List<InvoicePair>
            {
                new InvoicePair(new EInvoice{Version="1.03"},new TradeInvoice{Id=1,Value=100}),
                new InvoicePair(new EInvoice{Version="1.03"},new TradeInvoice{Id=2,Value=200}),
                new InvoicePair(new EInvoice{Version="1.03"},new TradeInvoice{Id=3,Value=300})
            };
​
        MemoryStream stream = new MemoryStream();
        Root.Serialize(data, stream);
        
        Console.WriteLine(Encoding.UTF8.GetString(stream.ToArray()));
        
        stream.Seek(0, SeekOrigin.Begin);
        

Cached Result

<?xml version="1.0"?>
<RootXML xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <e-Invoice>
    <Version>1.03</Version>
  </e-Invoice>
  <TradeInvoice>
    <Id>1</Id>
    <Value>100</Value>
  </TradeInvoice>
  <e-Invoice>
    <Version>1.03</Version>
  </e-Invoice>
  <TradeInvoice>
    <Id>2</Id>
    <Value>200</Value>
  </TradeInvoice>
  <e-Invoice>
    <Version>1.03</Version>
  </e-Invoice>
  <TradeInvoice>
    <Id>3</Id>
    <Value>300</Value>
  </TradeInvoice>
</RootXML>