Example use of the Unity 4.1 AirPlay API - gives a setup with the iOS device as controller of the remote display.

DualDisplay.cs

using UnityEngine;
using System.Collections;

/*
Runtime use:
To be available, AirPlay needs to be switched on for the device either via a native AirPlay UI component or
via the global AirPlay mirroring setting. Your options are:

A: Modify your Xcode project to layer a native AirPlay Cocoa control somewhere over your Unity content.

B: Enable global AirPlay mirroring on the device.
   - You will find this next to the global volume control - available when you slide the multi-task bar
     (accessible from double-home-button-click) to the right.
   - If AirPlay receivers are available on the network, you should see an AirPlay button next to the volume
     control.
   - Tapping it pops a target selector up and with a display capable target selected, toggle on "mirroring".
   - Returning to your application, AirPlay should now be available.

Though it is a more cumbersome user experience, I would recommend going with option B as long as it makes
sense, with an in-game description - to keep app simplicity.

If you want to test AirPlay, but do not have an AppleTV, I have successfully used AirServer for testing:
http://www.airserverapp.com
*/

namespace UnityAssets
{
	public class DualDisplay : MonoBehaviour
	{
		public Camera mainCamera, controlsCamera;
		public bool autoEnable = true;


		public bool Available
		{
			get
			{
				return enabled && Display.displays.Length > 1;
			}
		}


		public bool Active
		{
			get
			{
				return enabled && controlsCamera.enabled;
			}
			set
			{
				if (!value || !Available)
				{
					controlsCamera.enabled = false;

					controlsCamera.SetTargetBuffers (Display.main.colorBuffer, Display.main.depthBuffer);
					mainCamera.SetTargetBuffers (Display.main.colorBuffer, Display.main.depthBuffer);
				}
				else
				{
					Display secondDisplay = Display.displays[1];

					mainCamera.SetTargetBuffers (secondDisplay.colorBuffer, secondDisplay.depthBuffer);
					controlsCamera.SetTargetBuffers (Display.main.colorBuffer, Display.main.depthBuffer);

					controlsCamera.enabled = true;
				}
			}
		}


		void Reset ()
		{
			mainCamera = Camera.main;
		}


		void Start ()
		{
			if (mainCamera == null || controlsCamera == null)
			{
				Debug.LogError ("DualDisplay missing a camera reference");
				Destroy (this);
				return;
			}

			controlsCamera.enabled = false;
		}


		void Update()
		{
			if (Available)
			{
				if (autoEnable)
				{
					Active = true;
				}
			}
			else
			{
				Active = false;
			}
		}
	}
}

Has anyone managed to get this working with Image Effects on the Airplay screen, they refuse to work for me on Unity 2018.2.7