Yep, I know the first idea is regular compound turbo system except I am talking about pulling the primary turbo out of the circuit once desired (low for compund turbo) boost is achieved.
The second idea, dual scroll, does work.
The larger compressor and small high velocity exhaust section does work well for quick spool on a rotary, if you are mindful of a few things. You want to maintain high exhaust velocity without causing excessive backpressure to choke out our high overlap engines.
What I have found is critical is to have a VERY free flowing post turbo exhaust, then a large diameter exhaust wheel for the forceful rotary exhaust pulse to have great leverage on while maintaing a large flow area and last a decent size A/R with small volume to MAINTAIN velocity out of the engine instead of trying to RAISE velocity with a small tight A/R in a larger volume manifold/housing.
For instace, my current set up is just a 60-1 compressor w/ clipped "P" trim exhaust in a bored out, ported out stock S5 turbo housing/manifold and a 3.5" exhaust.
Data logs show positive manifold pressure literally off 1,000rpm idle, ~2psi by 2,000rpm and full 15psi boost before 3,500rpm.
Eliminating boost creep was the real challenge. After putting on a 60mm external WG did not fix boost creep I had to port out the stock velocity stacks in the manifold to slow the exhaust and port the runners to favor wastegate flow.
The problems w/ decent compressor, small exhaust, giant wastegate system like I have is that although the spool is great and top end is great the relaibility suffers when you have it under full load for long periods of time like back to back freeway pulls or endurance racing as the engine to turbo exhaust backpressure is so high the exhaust heat transfers more quickly to engine internals and turbo components.
Melted EGT probes, melted gaskets, failed manifolds and even warped apex seals result.
So, diverting the exhaust first through a second flow path once spooled and then finally through the wastegate once the limits of the now larger turbo are exceeded allows for lower engine to exhaust backpressure and greater reliability as well as slightly better top end power due to less charge dilution with overlap.
Look at it this way, even on a very high power rotary we are demanding full boost at ~3,000rpm to help low rpm power on our "oversquare" engines.
The problem is the engine is at best making 100RWHP of exhaust flow at 3,000rpm.
Now you want to at least quadruple it to 400RWHP of exhaust flow while maintaing the same boost over the next 5,000+rpm.
You need more volume in the system for the increased exhaust flow. I tried to do this by adding a huge wastegate, but this raises engine to turbo exhaust backpressure