So basically two sabers beats one in the star wars universe. This explains a lot because while you may not always NEED two sabers, there are countless scenarios where someone picks up another saber to gain an advantage in battle. Examples being, magus in the return trailer, maul vs sidious in clone wars, Kaden vs the inquisitor in rebels... the list goes on. Plus every npc in swtor with the title "master duelist" is dual weilding