I have heard of having 2 Turbos that connect to the same intake. One turbo is a small fan and the other a large fan. The produce boost at different RPM's. The idea is that by having the 2 turbos you can get the car to launch without turbo lag due to the turbos kicking in at seperate times. This is also a way to cut down temperature on the turbos because the smaller fan wouldn't be needed at the highend (spins too fast 14000 rpms or higher) and the bigger fan wouldn't be needed at the low end. (I could have these reversed). I could see a supercharger with a turbo addon as a way of elliminating lag, but the whole point of superchargers is that they usually provide boost the whole time (no lag).
I see above that this was done, but what pluses does this set up provide?
A well thought out turbo system will have virtually no lag. Also lag is almost non-exisitent on these size motors unless you are using thumper style tubos. Lag is more of a problem on small cube motors and thats why on my Supra it comes with a sequential turbo system. A pair of GT30's on a 346ci motor would spool more then fast enough to keep the biggest roots blower fan happy.