The generic CBVs defined within Django are designed to work with a single model. Likewise, a ModelForm is designed to work with a single instance of one model.
That doesn’t mean that other things can’t be done - only that those tools may not be the best way to do them.
See Create a new user, manually - #5 by typonaut as part of another discussion on this topic.
With just one field in play here, I’d use the Model Form for the model with the majority of fields being used, and add the field from the other model to that form. (You’ll have to initialize it when the form is initially rendered, and update it manually when the form is submitted.)
If you had a larger number of fields, I’d create two Django forms and render them within one HTML form. (They’re kept logically separate using the prefix attribute of the form.)