Example of using Win32 API to get specific window instance in Unity. This is useful for when running multiple instances of the same application, as the window handle can then be used to correctly position and size the different windows for multi screen use.

SpecificInstanceOfGameExample.cs

using UnityEngine;
using System;
using System.Runtime.InteropServices;
using System.Text;

public class SpecificInstanceOfGameExample : MonoBehaviour
{
    #region DLL Imports
    private const string UnityWindowClassName = "UnityWndClass";

    [DllImport("kernel32.dll")]
    static extern uint GetCurrentThreadId();

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern int GetClassName(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

    public delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam);
    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern bool EnumThreadWindows(uint dwThreadId, EnumWindowsProc lpEnumFunc, IntPtr lParam);
    #endregion

    #region Private fields
    private static IntPtr windowHandle = IntPtr.Zero;
    #endregion

    #region Monobehavior implementation
    /// <summary>
    /// Called when this component is initialized
    /// </summary>
    void Start()
    {
        uint threadId = GetCurrentThreadId();
        EnumThreadWindows(threadId, (hWnd, lParam) =>
        {
            var classText = new StringBuilder(UnityWindowClassName.Length + 1);
            GetClassName(hWnd, classText, classText.Capacity);
            if (classText.ToString() == UnityWindowClassName)
            {
                windowHandle = hWnd;
                return false;
            }
            return true;
        }, IntPtr.Zero);

        Debug.Log(string.Format("Window Handle: {0}", windowHandle));
    }

    void OnGUI()
    {
        GUILayout.Label("Window Handle: " + windowHandle);
    }
    #endregion
}