It is possible to have a two stage setup or have the gases flow through both.
Two stage setup: Supercharger off the line, turbo kicks in once its spooled. This setup is used to reduce turbo lag. They used to use these when engine tuning was limited to adjustment screws, but with computers these days and better bearings in turbos and general turbo design, you don't really get much benefit from this.
The other possibility is he may have the supercharger feeding the turbo. This I would not recommend to anyone running less than 130+ octane. This is usually only seen in extremely high horsepower vehicles, like those silly tractors pulls. You can get some serious pressures out of this setup way way more than a Mini engine could handle.
My typhoon has a small turbo on it, feeding a 4.3l block at an 8:1 compression; factory boost pressure is about 15psi. I know people with aftermarket turbos running about 25-28 psi but that is really at the high end of street driven. A mini engine probably couldn't handle more than 10psi in stock form maybe 15 in all out race form.
I wouldn't believe it on a mini, and I didn't even address the space consideration. He would be better off with just a turbo, tuned right, lots of intercooling, some waterspray, drive it only when the outside temp is 0deg Celsius.. etc. I would guess the supercharger is probably hooked up to his a/c system to keep all the hot air out of the car when he drives it!
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