After getting involved in both the sites; I think I now have enough experience of both, to make a statement.
The DataScience SEData Science site is about the problems and doubtsquestions which data scientists encounter on a regular basis, like "howHow do you run neural networks on a cluster efficiently?" and "How do I efficiently setupset up a MachineLearningmachine learning process on a server?", etc. do not belong to the CrossValidatedCross Validated or the StackOverflowStack Overflow sites. They can be only answered inon the DataScienceData Science site.
As of now, a major chunk of DataScienceData Science SE's questions are on-topic with posts on CrossValidated;Cross Validated; 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 DataScienceData Science SE and the CrossValidatedCross Validated sites are completely different from each other and serve their purposes very well.
A similar discussionA similar discussion on the CrossValidatedCross Validated meta.