Sequential is mostly preferred because of its near lagless nature. Small turbo spools up fast but can't give high boost. So this kicks in at low rpms to give instant response and loads of low end torque. Around 3-4k rpms, the small turbo can't supply enough airflow and pressure. Thus the big turbo kicks in. The separation of exhaust gases to the small or big turbo is controlled by a vane. As the rpm goes up, less and less is directed to the small turbo and more towards the big turbo. Usually it attains full boost by 4k rpms. RB26DETT attains full boost by 3.8k rpms (read from autocar long time ago autocar). So its like the best of both worlds. Instant response and able to rev high with a lot of power.
v-engines usually uses parallel. They have exhaust gases from one bank of cylinders spooling turbo that charges the intake air for adjacent bank of cylinders. This creates a natural regulating mechanism that keeps the engine stable. There won't be one bank of cylinders making more power than another in this setup. If 2 small turbos are used in this system, you get instant response but not high power. If 2 big turbos are used, you feel constipated while starting but pee in your pants when both the turbo kicks in.