鹬蚌相争, 渔翁得利

yù bàng xiāng zhēng, yúwēng dé lì

[two dogs fight for a bone, and a third runs aways with it; Parties in a quarrel are both losers] 比喻双方争利、 互不相让, 结果两败俱伤, 反而使第三者得利