Also have a look in the performance section of this list.

The parallel turbine / parallel compressor was used on the aftermarket M103 / M104 inline 6. This was supposed to give less boost lag. It is also used on some V engines as it makes for better packaging.

The parallel turbine / series compressor would be for higher boost pressure, just like a 2 stage shop air compressor. However, I see an imbalance in exhaust pressures between cyl 1-3 and 4-6 that would lead air flow and mixture issues between the two banks.

Typically, the turbines are in series for good balance. It is possible to have series / series with 4 turbos as each bank of the engine is treated as a separate engine.