This idea is a very old one and was actually used in a few production cars back in the 80's. Since that time it has shown up on a few 1st gen toyota mr2's and the meguires integra. In the long run the whole idea ends up costing more then it is worth and becomes more of a bragging rights " i have a twin charged car". In reality a properly tuned turbo/ or supercharged car will ended up running just as well or better.
To break it all down. In order to run a car like this you need a smaller supercharger and a larger turbo. So the motor has to be built and sturdy enough to handle high boost. There is no point in a system like this if you plan to run 8 psi since you can get that out of just a super or a turbo. You also need a clutched supercharger so that it can disengage when the turbo is spooled up. With this said. the superchager still needs to be able to handle the rpm's when the turbo begins forcing air through the supercharger.
Lastly, and the hardest part is the tuning. a system like this requires a stand alone management system, and someone with the knowledge to properly tune this.
All of this can be done, but it takes a lot more then simply bolting a turbo on underneath your neuspeed supercharger. All of these parts will be one of pieces and will cost you a hefty amount of money. In the long run, you would have been better off doing your research and purchasing a properly sized turbo with a equal length tubular manifold and air/ water intercooler. Running a GT series garret turbo would eliminate a lot of the lag associated with a turbo and allow you to run a slightly larger turbo for added boost. With an air/water you could run a short piping route and have a very responses and quick car to drive.
Now, if this is something you would really like to see, all the power to you, in my opinion it is simply easier to build a proper supercharger or turbo system from the beginning.
