By running two generators in parallel, you reduce the output each one is required to put forth. For example, suppose you calculated out the load to be 1,000 watts. Running just one generator, it will have to produce 1,000 watts of power. By running two generators in parallel, the wattage output is divided by two, so each output is 500 watts. A 500-watt generator is lighter, smaller and more transportable than a 1,000-watt unit. If the generators are mounted on a wind turbine, less bracing is needed on the tower.
If you were using just one generator, the entire charging system would have to be shut down if it failed. With two generators, if one fails the system can keep running until the faulty one is repaired or replaced. The power output is reduced by half, so conservation steps have to be enacted while the system is under repair. Even so, the entire system can keep running.
Two generators can run during peak power times, one can be cut off during low power times. For marine systems, this is a critical advantage. For example when air conditioning is needed, both can be run. When temperatures cool down, only one generator is needed, and the other one shut off. This leads to fuel savings while the ship or boat is in mid-voyage, with no source of fuel nearby.
Students at the University of Michigan were requested to design a small hydroelectric power system for Alaska in 2007. To extract the maximum power from a flowing stream, they ran two generators in parallel. By doing so, they were able to extract out twice the power from the same stream. The same principle can be applied to wind turbines. By placing two turbines side by side, twice the power is extracted from the same wind area.