I had a jza80 twin turbo supra with the standard turbos running around 1 Bar (14.7 psi). This system worked well and you could tell when the larger turbo came on boost.This system is simple, but because I'm lazy I will let toyota say it in there own words.
Turbo System consists of 2 turbochargers. See Fig. 2. While No. 1 turbocharger provides boost pressure at low engine RPM and engine load
conditions, both turbochargers together provide boost pressure at high engine RPM and engine load conditions. System is equipped with a
Charge Air Cooler (CAC) to cool turbocharger intake air. Cooling of turbocharger intake air increases air density, resulting in increased
engine output. An intake air control valve is located on No. 2 turbocharger.
See Fig. 2. An exhaust by-pass valve and exhaust gas control valve are used in the turbocharger system. See Fig. 3.
During low engine RPM, intake air control valve, exhaust gas control valve and wastegate are closed and only No. 1 turbocharger
provides boost pressure. When boost pressure reaches a predetermined level, exhaust by-pass valve opens. When exhaust by-pass valve opens,
exhaust gas is delivered to No. 2 turbocharger and No. 2 turbocharger operates to provide additional boost pressure.
During high engine RPM, exhaust gas control valve and intake air control valve are opened. This allows No. 1 and 2 turbochargers to
provide boost pressure. At this time, even if exhaust by-pass valve operates, it cannot affect boost pressure, since it is downstream of
No. 2 turbocharger. During high engine RPM, the wastegate controls boost pressure of entire system.
The Engine Control Module (ECM) controls the wastegate, exhaust gas control valve and exhaust by-pass valve operation by using
a Vacuum Switching Valve (VSV) for each component. The ECM uses various input signals for determining VSV operation. The VSV controls
the pressure flow to an actuator for the wastegate, exhaust gas control valve and exhaust by-pass valve.
Courtesy of Toyota Motor Sales, U.S.A., Inc.
See like I said simple.