Here is how they might have done this -
Well that's very simple, you need to create a Main application window and then write your code in such a way that by any IPC technique when you get Handle for top most window you create new window as child window of this Main application window.
In C++, Win32 API you can create top main window using CreateWindow/Ex function, when you want to create a child window with different process create a new process of the same executable. Then pass this HWND to this process using any of the IPC method or simply in command line arguments to this new process, and design your code in such a way that when you get handle then you create new window using same CreateWindow/Ex api with this handle as parent handle and there it is. You get your SingleWindow multi process application. I will attach a sample code for this later.. :)
TIP: By SingleWindow I mean a single main application window with which user usually interact.