SecondActivity.cs

using Android.App;
using Android.Content.PM;
using Android.OS;
using Android.Support.V7.App;
using Android.Views;

using Xamarin.Forms.Platform.Android;

[Activity(
    Theme = "@style/MainTheme",
    ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation,
    ScreenOrientation = ScreenOrientation.Portrait)]
public class SecondActivity : AppCompatActivity
{
    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        var page = new SecondPage().CreateSupportFragment(this);
        SupportFragmentManager
            .BeginTransaction()
            .AddToBackStack(null)
            .Add(Android.Resource.Id.Content, page)
            .Commit();
    }
}