I am not an expert, but here is my understanding. There are a number of reasons to run twin turbos or a turbo/supercharger combination on small motors. The classic example of a twin turbo a small motor was the RX-7.
With only one turbo you must choose small, medium, or large (I know this is a drastic over simplification.) With a small turbo you get fast spool up for more power down low, but it poops out at high volume boost production for the big HP numbers. A big turbo makes the big power but it takes longer to spool up so low RPM oomph is missing. Many FWD guys use these on small engines because they have no traction anyway so it helps them get out of the hole. Once they hook up the big turbo kicks in and they are off. This is why you see some big HP numbers at high rpm and low torque figures at the low end and mid range.
Medium is a obviously a compromise. The variable vain turbos of the late 90s were an attempt to have a turbo act small at low RPM and then act large at high.
The RX-7 used sequential turbos in small and larger size to get the best of both worlds.
In the mid 80’s Fiat and Lancia used turbo/supercharger combinations successfully in World Rally cars during the infamous HP race in the Group B rally cars. They were known as the Killer Bs. Lancia actually won the championship with this combination in their Delta S4. http://freespace.virgin.net/shalco.com/lancia_S4.htm
There are other examples, but I have written enough.