in the solution explorer (right side window), rightclick copy mainActivity.cs, right click paste it on projectname.android
then rename it (new name.cs)
change the declaration to
[Activity(Label = "SecondScreen")]
public class SecondScreen : Activity
you can also change the icon, save all
in the main activity summon the new activity with:
button1.Click += delegate { StartActivity(typeof(SecondScreen)); };

2.2 manipulating the newly added activity UserInterface technique :
in the layout folder add SecondScreen.axml (in my case SecondScreen.axml) by copypasting the main.axml
add SetContentView(Resource.Layout.SecondScreen);
here is how my code looks like now :

Code:

using System;
using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;

namespace App15
{
    [Activity(Label = "SecondScreen")]
    public class SecondScreen : Activity
    {
        int count = 1;

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.SecondScreen);

            // Get our button from the layout resource,
            // and attach an event to it
            Button button = FindViewById<Button>(Resource.Id.MyButton);

            button.Click += delegate { button.Text = string.Format("{0} clicks!", count++); };
        }
    }
}

set the second screen.xaml background image to one you add to the resources drawable
now the SecondScreen.axml is the UI of the SecondScreen.cs you added bwahaha !