Yeah, now you think I'm messing with you: yesterday was subtractive light, now I'm saying there's something called additive light. What's more, they're totally unrelated concepts.
Subtractive light is a process/technique of shaping light by blocking light sources. You can do it by positioning your subject under the shade of a tree on a bright day (or the shadow of a lamp post/building/whatever). Or you do it on an overcast day by moving them under a tree or next to a building. On overcast days it reduces drab by making the light more directional from the front.
Additive light is for technical geeks.
We should start with the question: if you're using a key and fill light, is the fill adding to the amount of light produced by the key (since they overlap)? And if so, by how much?
The answer is yes. Light molecules add up. The answer to "how much" is that all math: if your fill is 1 stop under your key, it will add 1/2 a stop to the key light where they overlap. A 2 stop difference between light sources adds 1/4 of a stop to the key, and so forth. Why the hell would you want to know this? Well, if you are using a light meter (and I recommend you do in certain situations), then you should know that your key light measurement isn't precise until you add in your fill. It's an important consideration in studio.
So, if your key light is at f/8 and your fill is f/5.6, then your aperture should be set to f/9 for the correct exposure (f/9.5 actually, but lenses don't do that). The difference between f/8 and f/5.6 is one stop, so the brightest part of the image will be f/9.5 according to additive light theory. If you're working with a 2 stop ratio, then the correct aperture is still f/9 (f/8.75, technically). Which tells us that as you increase the ratio between your key and fill, the amount you need to compensate for becomes negligible.
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