Gee did I just read SE v Multi eng? Now where talkin' as that's right up my alley:-)
We've covered the main reasons as to why 2 drivers.
Redundancy, pilot incap, 4 eyes are better than two, work load, system complexity, currency,insurance, cost & plain simple too dangerous with just one driver, all revolves around the two key words with flying, COST & safety, typed like that as it's easy to see where the priority lays.
Wmk2