The basic reason V8 setups use twin turbos is because they have two separate exhaust manifolds on opposite sides of the engine. You can't run a turbo off the exhaust of just one bank of cylinders, and let the other flow free. And the plumbing to bring both exhaust streams together and then route it to a turbo and route it back to the intake would be so long that it would generate enormous turbo lag. It just doesn't make any sense. On the other hand there are plenty of single turbo setups for slant six engines, where the exhaust and intake are on the same side of the engine, making it simple.
