this set up is typically used for both low rpm and high rpm response.
the small turbo spools up at low rpm to give good low down power till the big turbo spools up for mid range and hige rpm.
Works ok on series 6 mazdada rx7 and early model soarers, the smaller turbo generates a lot of heat though and in both cases tends to craock the manifolds after about 50,000kms.
Its like the 4agze supercharger and turbo kit, works well in theory but could work better in real life.