How is it possible to build a cable car line from one mountain to?
Same way you string the cables for a suspension bridge - you first have to run a pilot line from one side to the other, somehow. Maybe you use a helicopter. Maybe you simply have someone carry the end of a lightweight line to the far point. However its done, running the pilot line then gives you a way to pull successively heavier cables across, until you get to the point where you can start to use the cables to support at least temporary structures from which the workers can continue to the construction.