Unity editor tool to create dummy Unity projects to allow a single project to be open in multiple editor instances.

UnityProjectJunctionTool.cs

// gist by Roystan (IronWarrior): https://gist.github.com/IronWarrior/005f649e443bf51b656729231d0b8af4
// Video demo: https://twitter.com/DavigoGame/status/1300842800964018178
//
// CONTRIBUTIONS:
// Mac and Linux support added by Creature Coding: https://creaturecoding.com/
//
// PURPOSE:
// Unity does not permit a project to be open in two different editor instances.
// This can be frustrating when building projects with multiplayer networking,
// as it would require you to create a build every time you wish you test your netcode.
// A workaround would be to duplicate the project, but this would require the duplicated
// project to be updated each time a change is made.
//
// Instead of duplicating the project, we create a new directory and populate it with
// junction links to our existing project's Assets, ProjectSettings and Packages directories.
// https://docs.microsoft.com/en-us/windows/win32/fileio/hard-links-and-junctions#junctions
// The contents of these junctions are identical, but Unity will treat the directory containing
// them as a new project, allowing us to have multiple instances of our project open.
//
//
// TO USE THIS SCRIPT: 
// Place it in your Unity project in a directory called "Editor". A new menu will appear
// on the top bar called "Unity Project Junction". Click it and Create Unity Junction Project.
// You can now open this "new" project in a separate Unity Editor instance.

using UnityEngine;
using UnityEditor;
using System.IO;
using System.Diagnostics;

public static class UnityProjectJunctionTool
{
	[MenuItem("Unity Project Junction/Create Junction Unity Project")]
	private static void CreateJunctionUnityProject()
	{
		DirectoryInfo projectDirectoryInfo = new DirectoryInfo(Directory.GetCurrentDirectory());

		string junctionDirectory = $"{projectDirectoryInfo.Name}-Junction";
		string junctionPath = Path.Combine(projectDirectoryInfo.Parent.FullName, junctionDirectory);

		if (Directory.Exists(junctionPath))
		{
			if (EditorUtility.DisplayDialog("Junction already exists",
				$"Junction project directory already exists at {junctionPath}.\n\nWould you like to create another junction project?",
				"Cancel", "Yes, create another"))
			{
				return;
			}
			else
			{
				int junctionIndex = 1;

				do
				{
					junctionDirectory = $"{projectDirectoryInfo.Name}-Junction-{junctionIndex}";
					junctionPath = Path.Combine(projectDirectoryInfo.Parent.FullName, junctionDirectory);

					junctionIndex++;
				}
				while (Directory.Exists(junctionPath));
			}
		}

		DirectoryInfo junctionDirectoryInfo = Directory.CreateDirectory(junctionPath);

		string linkAssets = Path.Combine(junctionDirectoryInfo.FullName, "Assets");
		string linkProjectSettings = Path.Combine(junctionDirectoryInfo.FullName, "ProjectSettings");
		string linkPackages = Path.Combine(junctionDirectoryInfo.FullName, "Packages");

		string targetAssets = Path.Combine(projectDirectoryInfo.FullName, "Assets");
		string targetProjectSettings = Path.Combine(projectDirectoryInfo.FullName, "ProjectSettings");
		string targetPackages = Path.Combine(projectDirectoryInfo.FullName, "Packages");

		CreateJunction(linkAssets, targetAssets);
		CreateJunction(linkProjectSettings, targetProjectSettings);
		CreateJunction(linkPackages, targetPackages);

		EditorUtility.DisplayDialog("Complete", $"Created junction project at {junctionPath}.\n\nYou may now open this new project in a separate Unity Editor instance.", "Ok");
		
		EditorUtility.RevealInFinder(junctionDirectoryInfo.FullName);
	}

	private static void CreateJunction(string link, string target)
	{
		if(Application.platform == RuntimePlatform.WindowsEditor)
        	{
			LinkWinOS(link, target);
	        }
		else if (Application.platform == RuntimePlatform.OSXEditor || Application.platform == RuntimePlatform.LinuxEditor)
		{
			LinkMacLinOS(link, target);
		}
	}

	public static void LinkWinOS(string link, string target)
	{
	        string command = $"/C mklink /J \"{link}\" \"{target}\"";
        	Process.Start("cmd.exe", command);
    	}

	public static void LinkMacLinOS(string link, string target)
	{
		string command = $"ln -s \"{target}\" \"{link}\"";
		
		var proc = new Process
		{
			StartInfo = new ProcessStartInfo
			{
				FileName = "/bin/bash",
				Arguments = "-c \"" + command.Replace("\"", "\"\"") + "\"",
				UseShellExecute = false,
				RedirectStandardOutput = true,
				CreateNoWindow = true,
            		}
		};

		proc.Start();
		proc.WaitForExit();
	}

}

Nice job on this, I've been doing this manually for a while now, but this certainly improves the workflow. I went ahead and added support for Mac and Linux which you can find in a modified version here. Feel free to pull those modifications into this gist if you would like.

Awesome! Added it into this Gist, attributed the changed to you and linked to your website (lemme know if there's a different way you prefer attribution).