There was a talk a couple years back at Djangocon where a company was using both flask and Django together. It was a transition for them from flask to django,

There was a talk a couple years back at Djangocon where a company was using both flask and Django together. It was a transition for them from flask to django, but it wasn't hard to do. Probably doesn't make a lot of sense for most projects, but it is possible because Django doesn't get in the way.