Multiple users developing for iOS simultaneously on a Mac using RealVNC
Simple solutions

Virtual Machines

You can create OS X virtual machines and connect to them instead of the main session. Apps that can do this are for example: VMware, Virtualbox etc.

VPN clients spawning sessions

I do not know about Windows, but on OS X when you connect via Screen Sharing.app there is a possibility to create new session remotely. Maybe someone from the "Windows world" knows about that kind of software and can share knowledge about it.

Advanced solution

If You have a remote access app, which is able to connect also to the login window (the place where you choose the user and provide the password) - you can use a little hack:

Run the process with those lines (can be done via SSH):

CGSSessionID outSession = 1;
CGSCreateLoginSessionWithDataAndVisibility(&amp;outSession, NULL, false);


This has to be compiled to the executable - it's written using C language and private API (more info on stackoverflow).

This will create separate GUI session in the background - then connect to the server running on the login window and connect to this "new" session.

NOTE:


The easiest way is to use VirtualMachines.
One of the app, that I know which can log into login window is NoMachine ( I was a developer of this app couple years ago and multiple sessions on OS X were possible, but that was proof of concept. Maybe this works out of the box right now).
I do not know if this CGSCreateLoginSessionWithDataAndVisibility hack still works in the newest OS X (I made this on Yosemite). And You will have to have some knowledge about programming and GUI sessions on OS X in order to use it properly. There could also be a problem with "disappearing" session (fun to experiment with this, though).