OK I will try to explain as I am not good at this.
First I would give you an example just to prove that it works. Take your example but change the flashes to 2 identical units with the GN of 56. Using the formula it came out as 80 for the combined GN. Which give you f/8, 1 stop more than with 1 flash.
The reason has to do with the inverse square law that states that the illumination at the subject is inversely proprotional to distance squared. And so if we are to keep the same amount of illumination but increasing the distance then the flash power has to be increased proportionally with the distance squared. The aperture number in similar fashion that the intensity of the light fallen on the film is proportional to inverse of the aperture squared because light intensity is proportional to the area of the lens opening not its diameter. With this 2 factors the square can be removed and form a simple relationship between distance and aperture. However, when we deal with power we have to put the square back before we can add or mutiply. I hope it helps because like I said I am not good at this.