After getting involved in both the sites; I think I now have enough experience of both, to make a statement.
The DataScience SE is about the problems and doubts which data scientists encounter on a regular basis like: "how do you run neural networks on a cluster efficiently?" and "How do I efficiently setup a MachineLearning process on a server", etc do not belong to the CrossValidated or the StackOverflow sites. They can be only answered in the DataScience site.
As of now, a major chunk of DataScience SE's questions are on-topic with posts on CrossValidated; and it is just because due to the fact that statistics make a major role in data science, so it is and will be a common happening across both the communities.
However, the DataScience SE and the CrossValidated sites are completely different from each other and serve their purposes very well.
A similar discussion on the CrossValidated meta.
