My $.02 says don't do it. Carpeting is strrrreeatched when it's installed. If you lay it on top of other carpet, the carpet below will 'give' after time, and you'll end up with saggy carpeting on top. Also, as usual, it may void the warranty. Carpeting needs to have a certain underlayment when it's installed.
As for carpet on wood, it's done all the time. If you're referring to a 'throw' carpet or an oriental rug on top of a wall-to-wall carpet, that's ok. It is done all the time too. But if you're referring to wall-to-wall on top of wall-to-wall, I've never heard of it.