If all components weight the same (shaft/wheel ect) and have the same wheel flow efficiencies, a single turbo is the most efficient way to compress air in any system. This is straight from the mouth of as garret engineer. In lay terms, you can get the tightest tolerances between the wheel/housing, and more turbos just augment that loss. Most twin systems are done for packaging in the OEM world, or sillay airflow needs int he aftermarket that make a single not an option.
And besides- its been done before.
http://67-72chevytrucks.com/vboard/attachment.php?attachmentid=283935&stc=1&d=1184886430