You can use both of them on the same project. The reason is that CherryPy is primarily a web application server, and Flask is primarily a web framework.
(I'm not sure why you would want to do that, because Flask comes with Werkzeug, which is better integrated and I trust its author a bit more.)
Also, I've never seen a better documentation than Flask's.
> (I'm not sure why you would want to do that, because Flask comes with Werkzeug)
Well for one, Cherrypy-the-web-server is actually good enough for production. There is much to be said for the benefits of having the same deployment environment in development and production.
The development server is not intended to be used on production systems. It was designed especially for development purposes and performs poorly under high load. For deployment setups have a look at the Application Deployment pages.
(I'm not sure why you would want to do that, because Flask comes with Werkzeug, which is better integrated and I trust its author a bit more.)
Also, I've never seen a better documentation than Flask's.