Xamarin.form setpage not worked second time

I create new mobile app using xamarin form. I have to create two pages login screen and home screen.
I get sample from Here but when i call setPage method second time in my xamarin android activity it doesnt do any thing

       public class MainActivity : AndroidActivity, LoginManager
        {
         protected override void OnCreate(Bundle bundle)
       {
        base.OnCreate(bundle);

        Xamarin.Forms.Forms.Init(this, bundle);

        SetPage(App.GetLoginPage(this));
      // SetPage(App.GetLoginPage(this));


    }
    #region ILoginManager implementation
    public void ShowMainPage()
    {
        SetPage(App.GetHomePage(this));
    }

    public void Logout()
    {
        SetPage(App.GetLoginPage(this));
    }
    #endregion
}`