Ordinary people facing an assault by two similarly powerful opponents would choose to retreat.
After all, if you do not have the confidence to win against one person, why would you force yourself to stand your ground against two? Isn't that simply courting death?
But Su Xin was different. His martial arts was the evolution of the extreme of battle. Rather than turning and fleeing, ceding the initiative to others, it was better to seize the initiative for himself!