So let me first say that there could be colloquial usage where they are the same, or very similar. However typically "back stage" is a location, and is specific to a performance. It's a place one can go or be. "Behind the scenes " could include that, but could also include other aspects of how a performance or other product is created. So "behind the scenes" of a play could show backstage, but could also show how the performance was funded, how actresses lived while preparing, or other interesting stories.

Typically "backstage" is used to literally mean the activities (or the area) behind the stage of a play or behind the set of a movie or TV show; "behind the scenes" can be used the same way, but it also often used figuratively to mean all of the work that is done by support staff to make something large or important happen. For example, you could say that when astronauts go into space, there is an enormous team of technicians and engineers working behind the scenes to make the mission go smoothly. The astronauts are very visible to the public (i.e. you see them on TV) but the other people aren't often shown, which is why the phrase "behind the scenes" is used, because those people are not as visible to the public.