It is all about capacity and available space.

With twins you choose 2 turbos that have a combined capacity for both the exhaust flow and intake flow at the engine speeds and power output you wish to maximize. A single with the same desired flow and power would have the same total capacity as the two smaller turbos. The exception being if you want to sequence the turbos or stage the turbos.

Sequencing means that at low rpm only one turbo is active and the other is brought in at a higher rpm when more total capacity is needed.

Staging is often confused with sequencing but is actually one turbo feeding another. This allows more total boost than one turbo is capable of producing. This is almost never seen on a gasoline engine and is almost exclusively for ultra high output deisels which can top 200psi with 3 staged turbos.

Back to the question. One turbo is almost always cheaper than two assuming the same type turbo is compared. Packaging is always an issue as the hot side of the turbo and the exhaust pipes to and from it are VERY hot (exception being a remote turbo installation). These hot pipes must be routed to provide good flow without causing heat damage to belts/hoses/electrical/ect.

Untimately the engine doesn't care or know if one or two (or more) turbos are used. An optimized system will always work better than a compromised one regardless of turbo count.

Bling wise you can always say that your engine is TurboSupercharged. This is the actual correct term for any "turbocharged" engine.

 

Good turbo information

http://www.mazdausa.com/MusaWeb/disp...Protege&bhcp=1

from the above, an example of turbo HOT