Two flashes on manual, I know the guide number for both of them, the will both light the same area (more or less, they are both set on wide and will fill more than the angle of view the lens gives), can I just add the guide numbers to get the correct exposure?

Ex, distance 10 meters, flash 1=GN 45, flash 2= GN 64, hence I would get an f/stop of roughly (109/10)~=f./11 ?