My instinct is that using two different databases in one app adds a lot of complexity, but it's not clear to me that if you need to pick one database that SQL is a better choice than a graph database. It really depends on the application. For many purposes, a graph database can do what a SQL database can do, because a SQL TABLE is very much like a graph collection of nodes. Of course, if you're starting with an app on a SQL database and just adding to it, that's very different from a "green field" project where you can pick technologies freely.