In terms of which angles of view the lens(es) will allow you to select, you're correct: the combination of an 18-55 lens and a 55-200 lens will let you choose from exactly the same angles of view as a single 18-200 lens.

However, I think you are missing a couple of important points, both of which are well covered in this answer: you'll get better image quality and a better maximum aperture with the 18-55 / 55-200 combination than you will with the 18-200. On the other hand, you'll be carrying around two lenses and having to change them - at which point, it's a personal decision as to which matters more for you.

The other minor point I'd pull up is that "zoom ratio" isn't really a useful concept for interchangeable lenses; as I think you've realised, it's a concept which makes sense only for lenses which start at similar focal lengths. As such, it's vaguely useful for most compact cameras, which tend to start somewhere in the 20-28mm (equivalent) range, but pretty much worthless for comparing most interchangeable lenses. See this question for more details if you want to.