ASP C# .NET Master Pages

Recently I encounter a few problems in my website. I use Master Page in order not to use same tools in every page. the problem is that I want to use 1 single page but using 2 Master Pages. how a single page can use 2 Master Pages?

So this is the answer.

private void Page_PreInit(object sender, EventArgs e)
{
    if (Session["userID"].ToString() != null)
   {
         this.MasterPageFile = MasterPage2.master"; //use master page number 2 when user login
   }
   else
    {





          this.MasterPageFile = MasterPage1.master";  //use master page number 1 before user login
    }
}