As things get more complicated, I will separate out by some kind of concept. Which can be something as simple as:

  /coreapp
    /forms_foo.py
    /forms_bar.py
    /models_bar.py
    /models_foo.py
    /views_bar.py
    /views_foo.py

You can do a more sophisticated module layout, but essentially something as straightforward as the above, all under a single "core" application. Prevents Django from fighting you when you want to work across the arbitrary "app" boundary.