It doubles the current to have two alternators. If the two alternators are the same (I don't know if they actually are) then they both give the ~25V output at 100 RPM at the same time. So what you gain is 2x more current because you have 2x the number of alternators. A lot like having two jars as pickles instead of one jar. It's not 2x as sour, just more pickles to eat.
Or, if he chooses to connect the two alternator output in series, then it would double the voltage, for the same current. That's like cooking with jalapeno peppers. If you put 2x the jalapenos in the sauce, it's twice as spicy, but you don't have enough to double the servings.