in the left part of the animation editor you can see each Motor6D that handles a part or mesh part in the animation editor, for example if the orientation of the Z axis of the torso is in 50, in the other animation you put it in negative, it would take a long time changing it in each frame, (copying and pasting + subtracting) but at the end they would be the same but in reverse, I recommend you to do it this way before someone gives you a better solution.