The first thing you should know is that in both cases overcharging is done by two turbochargers, which, being propelled by the exhaust air, compresses the intake air, and pushes it into the intake, thus making ignition more intense, and the result is the power boost for which the turbo engines are so famous.
The first engines which used turbochargers obviously had a single turbocharger and the fact that it had to be propelled by exhaust gases, created a moment of resistance in which these gases had to accumulate with a pressure high enough to overcome the turbine resistance, after which the power boost was generated. That moment of hesitation is known to everyone as turbo-lag.
With the evolution of the engines, the turbochargers improved, evolving toward maximum inertia, and minimal resistance, so that the smallest exhaust gas pressure causes them to move. Some of them have even adopted variable geometry in order to adjust this resistance and give the engine more flexibility.
All two turbocharged engines can also be called Twin Turbo, but not all of them are Bi Turbo. In other words, it’s like this – the Bi Turbo is also Twin Turbo, with Twin Turbo “coverage” being larger, spreading to include Bi Turbo.
So, any two-turbocharged engine can be called the Twin Turbo. The two turbocharger system may be sequential or parallel. Then, when it comes to a sequential two-turbocharger system, the two turbochargers are of different sizes, one of them being smaller and operating at lower speeds and a higher one, running at higher speeds. The circuits between them are usually separated by a valve.
In this way, the engine becomes more elastic at low speeds, but very efficient at high revs too.
Then, when the two turbochargers are parallel, they are identical, each of which is connected to half of the cylinders – to a four-cylinder block in a V8 engine, for example, or to one of three cylinders in a V6 engine.
Not just the V-shaped engines can have parallel systems the 6-cylinder or even 4-cylinder engines can route their gasses to two separate turbochargers. So, this configuration with two identical turbochargers is usually called Bi Turbo.
The benefits are similar to the system described above, and if the turbochargers are still low inertia, then the engine maintains similar elasticity at low speeds, while performance at higher rpm may be even higher.
Note: do not confuse BMW’s TwinPower Turbo with Twin Turbo. The Bavarian term also comes from the Twin Turbo idea, but in time it has also been applied to single-turbocharged engines that have two twin-scroll exhaust ducts that can move the turbocharger very easily, at different speed intervals. So, the turbocharger can be one only, but its mode of operation it’s doubled, hence retaining the TwinPower syntax.