If two or more people are at cross purposes, they do not understand each other because they are talking about different subjects without realizing this.(两人因未意识到彼此谈论的事物不同而)相互误解,相互矛盾,各说各的
I think we've been talking at cross purposes - I meant next year, not this year.我想我们谈论的是两码事——我指的是明年,不是今年。