Twin turbo question

Hi all I wanted to know if a mid 90's legacy twin turbo could be fitted to an early model RS legacy? Would the exhaust manifold need changing and if so would the manafold from the twin turbo RS fit? What controls the transistion between the two turbo stages? finally would a twin-scroll turbo with a custom manifold be a better option? my aim is to reduce turbo lag and/or improving low end torque whilst maintaining power at the top end. Any thoughs thanks Mike