I did this many years ago on a Toyota MR2 Supercharged with a turbo kit from HKS called a twincharger. Basicaly it added a very large turbo to the factoru SC.
The setup was similar to the above Lancia. An adjustable controller was provided to switch off the SC clutch at a preset turbo boost.
The hardest part is to provide a smooth transition from SC+turbo to just turbo. If you do not get it right when you cutover to just turbos the boost falls below when it was with sc+turbos, then SC kicks back in and you might get this oscillating sensation.
If you ran the turbo on its own you could count to 4 before any sort of acceleration could be felt. However with the SC the response was instantaneous with lots of top end power.