They are different skills. Not saying that it's hard to learn both, but there are standardized career paths that will lead you to be good at the modeling / techniques side of "data science" without learning much about software engineering. For example, studying math in undergrad. And there are certainly lots of people capable of productionizing messy R scripts without fully understanding the statistical ideas behind them. So I think, as a team leader, you are restricting yourself to some degree if you only hire people who can do both.