The shared D3D device part is actually the real magic here. Getting two render loops to cooperate on the same GPU context without stepping on each other is not trivial at all, especially with games that were never designed with that in mind. The fact that both worlds stay simulated simultaneously is pretty wild too, it's not just rendering one and pausing the other. GTA modders have been carrying the entire franchise's innovation on their backs for like a decade now lol.