That is not exactly true. The VADHO indeed runs a BMW Hydrogen fuel cell, but this produces power for a hybrid-electric drivetrain -- like all hydrogen cars. The V12 is likely to be conventionally-powered.
So why the tandem seating arrangement?
Ostensibly, this bubble canopy housing a single passenger behind the driver's seat is to allow for the powertrain's massive scale and the required gas tank for the Hydrogen powertrain. These tanks generally are only available in one cylindrical shape -- and housing a massive one that can avoid exploding in a crash is deeply challenging for car designers.
2007 ItalDesign VADHO