Just one more way of thinking about it:

F stops are worked out as the focal length divided by the aperture. e.g. 160mm focal length / 20mm opening = f8

If you extend the bellows to 320mm you are doubling the distance from the film to the lens (I am assuming at infinity focus the bellows extension equals the focal length). Therefore you are effectively doubling the focal length.

The new formula is: 320mm / 20mm (because the aperture stays the same) = f16

Therefore you need to give two stops more light than you would at infinity focus.