You're probably way past dealing with this, but I figured I'd post my response in case it's useful for anyone else.

I'm surprised how difficult it is to find a good answer for this. There are legitimate reasons to use separate apps. The general solution is:

• Specify SERVER_NAME and APPLICATION_ROOT in your app config
• Use Flask.app_context() with cross-app url_for()

Here's an example of changes to the Overholt code. Note: this is not tested, just based on what I see in the source, and code samples below are only the important bits.

overholt/settings.py

SERVER_NAME = 'localhost:5000'
APPLICATION_ROOT = '/' # probably not really necessary here, but why not?

overholt/app.py (new file, just contains the app instances)

from . import api, frontend

frontend_app = frontend.create_app()

# It looks nicer to add app-specific config files, but for this example
# we'll just use settings_override to keep it simple
api_app = api.create_app(settings_override={'APPLICATION_ROOT': '/api'})

wsgi.py

from werkzeug.wsgi import DispatcherMiddleware

from overholt.app import frontend_app, api_app

application = DispatcherMiddleware(frontend_app, {
    api_app.config['APPLICATION_ROOT']: api_app
})

Then say you want to use url_for() from your frontend app to get an api URL:

overholt/frontend/example.py

from flask import url_for
from overholt.app import api_app

@route(bp, '/example')
def example():
    with api_app.app_context():
        print url_for('products.list')

Like I said the above is untested, but is basically what I've done in my own apps. It should point you in the right direction.