You will get Session2. Session and Instantiation are 2 different things in WCF. I would recommend you read the following 2 blog entries that will help you understand the concepts of WCF sessions and instantiation.
http://www.dotnetconsult.co.uk/weblog2/PermaLink,guid,af6e6325-2e30-42e3-acb9-57e1363fa51e.aspx
http://www.dotnetconsult.co.uk/weblog2/PermaLink,guid,1efac821-2b89-47bb-9d27-48f0d6347914.aspx
you can also test this with a client. I changed your WCF method to return the sessionID and consumed it in the client.
static void Main(string[] args)
{
ServiceReference1.Service1Client client = new ServiceReference1.Service1Client();
Console.WriteLine(client.Regster());
Console.ReadLine();
client.Close();
}