Biturbo is the same as "Sequential Twin Turbo" .
Basically, you have 1 smaller turbo for lower RPM range, and a larger turbo for higher range. It helps greatly reduce turbo l…ag due to the smaller turbo can produce boost much earlier than a super single or twin turbo setup can do. At lower RPM, the smaller turbo spools and produces boost - then when you reach a preset RPM or boost level, a staging valve opens and directs air to turbo 2 (large one) and it now produces the boost for the engine. Usually, stage one turbo is turned off, or its boost is sent into the compressor intake of the stage 2 turbo to achieve a higher output pressure. .
Do not confuse Biturbo / Sequential Twin Turbo with "Twin Turbo". .
Twin turbo is having two EQUAL size turbo's. Each runs off its own cylinder bank in V or H type (boxer) engines, or off of a turbo manifold on inline type engines. Both turbos share the load equally in producing boost air. (Like a dual core processor in your PC - both run at same speed, sharing the load equally) So, turbo lag still effects this type of setup, as both turbos will spool at the same time. ( Full Answer )