This is a awsome bi-turbo engine but, it is not using sequential turbos... Look at the pic, turbos are the same size and it is clear that each turbo charges its own 3 cylinder bank...
Few words from press release:
"A further traditional turbo-engine wrinkle has been ironed out by BMW engineers: turbo lag. In the new engine, this has been completely removed, thanks to low inertia turbochargers. The driver need only brush the accelerator pedal to receive an immediate surge of power and performance, as the engine delivers an electrifying response. While turbocharger technology is almost as old as the internal combustion engine itself, in combination with cutting-edge high-precision direct petrol injection technology BMW engineers were able to write a new and exciting chapter in the history of forced-induction.

The foundation for all this power and torque is drawn from the exemplary characteristics of the magnesium-aluminum 3.0-litre engine which is rated at 195 kW/265 hp (as fitted to the BMW 130i and Z4 Roadster). This in itself is quite outstanding for a normally aspirated engine. Compared with the proven 3.0-litre normally aspirated magnesium-aluminium engine, the all-aluminium engine featuring new biturbo technology increases overall output by approximately 15 percent. Peak torque shoots up by an even more impressive 30 percent. The resulting 225 kW/302 hp and 400 Nm of torque deliver outstanding thrust and driving power all the way across the rev range.

The only way to achieve such an increase in power and performance with a normally-aspirated engine would be to significantly increase engine size, which would also mean a corresponding increase in weight and the negative effects on the car's overall balance. The BMW combination of turbocharger technology with high-precision petrol injection is a particularly efficient way to meet even greater demands in terms of output and torque. And for comparison, the new straight-six biturbo weighs about 70 kg less than an approximately equally powerful eight-cylinder normally-aspirated engine displacing 4.0 litres. That is about the same weight as a moderately sized passenger.

Direct petrol injection also offers a significant fuel consumption saving of about 10 percent compared with a similarly powerful turbocharged engine that uses regular fuel injection. Turbines made of high heat-resistant special steel can withstand temperatures of up to 1,050 *C and therefore do not need the cooling effect of extra air flow. Particularly under full load, this means a significant decrease in fuel consumption.

High-precision fuel injection allows an even more exact dosage of fuel as well as a higher compression ratio - ideal conditions for increasing engine efficiency and significantly reducing fuel consumption. All this is made possible by the central position of the piezo-injector between the valves. Fitted in this position, the innovative injector opening to the outside is able to distribute fuel in a conical burst ensuring particularly smooth distribution of fuel within the combustion chamber.

Apart from its low weight and class-leading fuel economy, the new biturbo engine is able to offer yet another unique BMW feature. This is supreme smoothness and refinement, precisely the virtue which has made BMW straight-six power units the benchmark for refined drive technology, acknowledged the world over. Indeed, the natural perfection of the straight-six layout gives the engine perfect balance in terms of free mass forces, avoiding vibrations even at high engine speeds.

A further important point is that this turbocharged version of BMW's six-cylinder comes with the same hollow, extra-light camshafts as on the normally-aspirated engine, as well as an electrically driven coolant pump operating only as required. Developing this new straight-six with biturbo technology, BMW is opening
up a new chapter in the long history of the turbocharged engine, which dates back to 1905 and the work of Swiss engineer Alfred Büchi."