A) Joining a turbocharger and a supercharger is way too complex and expensive. B) There's no point. If you want more boost then either get a pulley system or a new turbo with a larger compressor wheel. If anyone somehow put a "superturbo" in their car, the bottom end would probably end up exploding because of the high boost levels.
Do some more checking on that before you dismiss it the way you did. First off, there is actually a company working on a Twin-Charger set up. Many people have done this set up, enough so that it has it's own term. This is basically what is happening with a twin turbo car, and I'm sure if you go tell somone in an RX-7 or 3000GT VR-4 that their twin setup will make the bottom end explode they would disagree. You use the one to feed the other, making up for the lack of each ones disadvantages. Mercedes was using twin supercharger setups before any of us were born, well, most of us. The real issue is the expense and the complexity. If you just get a larger turbo, you're giving away gobs of low end power to get top horsepower. Ask someone with an Evo or a HUGE turbo setup about how their car handles in traffic off-boost. There is also limits to 'just change the pulley' as well, such as the problems that we're dealing with now. True, you can make as much power with just a turbo or a supercharger, you can also make as much power with a N/A engine that is built like a rock. It's all about what you want to do with it, and who is gonna give you shit when you pop your hood and show them your blown-turbo Saturn Ion Redline?