5e has no rules for two Samurai Fighters to fight simultaneously. They fight sequentially, and whoever reaches 0 hp first dies.It's fairly easy to resolve things simultaneously, much like two Samurai launching at each other and both of them falling down dead.
It doesn't say all of their movements and actions are synchronized. "Everyone in the family eats dinner at the same time" does not mean everyone at the table eats in perfect synchronicity, with everyone grabbing their forks at the same moment, cutting the food with identical, exactly timed knife slices, chewing and swallowing at the same moment, etc.In any case, you are free to do it however you like at your own table. But the book really does say they go at the same time.