significance不可数，不能用a
应该to cooperate或in the cooperation of
改写后：
It is a significant milestone for us and Daxin in the cooperation of the A+H listing business, the achievement of diversification and the expasion of our business in the mainland market.
It has a significance of milestone for Daxin and us on the cooperation of the A+H listing business, the achievement of diversification and the expansion of our businesses in the mainland market.
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