twin turbo setups have to be either
A: equal sized turbos
B: sequential, meaning valveson the intake and exhast piping so that all hot and cold gasses go where they needto, and not leak out the wrong direction anyplace
doing single exhaust that runs through a small turbo and then a large turbo will cause the large turbo to never spool because the small turbo chokes out the exhaust at higher rpm
if you want to seperate the systems,you need a valve that directs the exhaust to either turbo 1 OR turbo 2
you also need valves of some sort that keeps air from the compressor side of turbo 1 from back flowing through turbo 2 (and vise versa)
i have an idea on how to run twin equal sized turbos in a sequential setup, but it needs a bunch of custom machined precision parts