If you're going to store in a relational database anyway, why have the expense of having to develop and support both? It'd make more sense to try to either denormalize - with some support framework - or just use the SQL database from the beginning, the deciding factor being how important aggregate operations are to your app and simple questions like how much your NoSQL database costs to operate vs. your SQL database.

tl;dr: "Know your data access patterns and pick a good fit with your resources"