Can I use "over" instead of "in" in the following sentence?
The first talks between North and South Korea in/over two years have produced an immediate dividend
No, it wouldn't be correct.
The first talks between North and South Korea in two years... means that there haven't been any talks between North and South Korea in the past two years, so that's fine.
The first talks between North and South Korea over two years... doesn't really make sense in the context of what you are trying to say. It would mean that the talks have been going on for the past two years.
Maybe what you are trying to say is
The first talks between North and South Korea in over two years have produced an immediate devident(?).
The in over there means that there haven't been any talks between North and South Korea in more than two years.
And by the way, there is no such English word as devident so do you mean effect?