Is an 18-200mm the same as two separate- 18-55 & 55-200mm?

I know this is probably a really stupid question but I was thinking of buying a new 55-200mm lens for my Nikon but from SIGMA (my friend has one and they are so much clearer & better quality), but then I saw the 18-200mm by SIGMA, and was wondering if I sold both my lenses and just bought the 18-200mm then I would only need to carry around one lens, would it be the same as having two separate lenses?

With the lenses you mention, yes, it would be pretty much the same thing. The whole purpose of the superzooms is to allow you to use a single lens for just about everything, instead of carrying around a bag of lenses and changing them constantly.

On both nikon lenses in question the widest aperture at long end is f-5.6 where as on sigma lens it is f-6.3 which would translate into 1/3 stop less on sigma lens. A minor issue!

Nikon's 55-200 outperforms the excrement out of the sigma 55-200. IF you don't get that you either had a dud or you need a serious skills boost.

A 18-200 covers the same range as those two lenses but will have different strong and weak points.

In terms of zoom ratio, yes. But in case of image quality no, its not the same thing. The 18-200mm would have more dispersion and less sharpness due to increase number of lens elements to cover the extended range of focal length than the 18-55mm and 55-200mm.there's a reason why most photographers choose to have multiple lenses with small range of focal length covering a wider range in total than fewer lenses with a wide range. But then again, at the end of the day there are more important things in photography like composition and subject matter. And the simple fact of versatility of having a single lens to cover all your needs may actually serve your purpose better letting you concentrate on the other aspects of photography instead.atleast for beginners the 18-200mm would be a better option.