That crossover tube is needed to equalize pressure. Dan is using a small 8rib? belt that has the potential to slip. If one slips and the other doesn't, then you could have uneven pressure distribution which isn't easy on the engine.
Two superchargers can fill the engine easier than one spun at a much faster rate. This will also cut down on the belts propensity to slip under boost.
Note the guys at Procharger (ATI) claim that their B600 (a more efficient and noisier unit) requires about 75 hp at 10 lbs of boost on a 350 chevy. Imagine how much horsepower would be required and the strain put on the belt if you tried 10lbs of boost on a 488 cu in engine!