I thought it was clear that i meant at the same time

In sci-fi I dont think the actor list means anything. Darth Vader is voiced by James Earl Jones in almost every scene where he is wearing a mask Darth Vader is physically acted in the suit by David Prowse in most scenes Darth Vader is physically acted in the suit by Bob Anderson in the intense lightsaber fight scenes Darth Vader is physicially acted and voiced in the suit by Sebastian Shaw when he is unmasked, dies and then shown as a jedi ghost. (ghost image replaced in later releases by ghost hayden christensen) Given that the Speaker is also a masked character and is a 3D rendering, I think they can do whatever they want, whenever they want.