Portmanteaus are typically formed by combining two words. While you could create one for French and German, it would likely be quite specific to the context you have in mind.
Here are some possibilities:
* "Franco-German" is a common term that already exists, though it doesn't quite fit the "portmanteau" definition.
* "Deutsch-Français" combines the German and French words for "German" and "French".
* "Francgerm" or "Germfrance" are creative, but potentially confusing.
Ultimately, the best portmanteau would depend on what you're trying to communicate.