That's usually correct, but there are several other factors involved, primarily the strength or power of the two lights. For example, if you shine two lights on the same spot, the light intensity will increase, but this is where the variable come in. Two 'identical' lights focused on the same area will not double the light (one f-stop), instead the light will be increased only one and one half times. That ratio is further confused by lights of vsrying strengths, differing reflector shapes, light to subject distances, as well as the use of umbrellas, softboxes and other light modification devices. With 'hot' lights, it's further confused by the relative age and color of the bulbs.
Yes, when more than one light is used the lights should be metered individually, one at a time while the others are turned off. That establishes their individual areas of converage and where shadows fall, but also their relative intensities, thus showing their 'ratios'. Finally a final light reading should be taken with all lights on to establish the working aperture. That should be done with a hand held incident flash meter - never a conventional light meter. No conventional light meter can measure that 1/500thor 1/1000th burst of light. An in-camera meter works a bit differently, usually by measuring the light to subject distance for a camera mounted light. It can't begin to handle multiple lights.
There is absolutely a learning curve to be considered when learning studio lighting. I found all of the books to be terribly confusing and not very helpful. Since you are finding the basics to be confusing (as I did), my best advice would be to find someone in your area to sit down with you and help with the lighting basics. I've done that with several people and they generally found it to be helpful. Second to that, find a good "teaching" workshop in which lighting techniues are both explained and demonstrated. That's what I did and it's been just a matter of gaining experience since then.
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