Formal:
* Is it possible to communicate in both English and French?
* Would it be feasible to communicate using both English and French?
* Is it possible to switch between English and French during our communication?
Informal:
* Can we talk in English and French?
* Is it okay if we use both English and French?
* Can we mix English and French when we talk?
More specific:
* Can you understand me if I speak in English and French?
* Is it possible to have this conversation in both English and French?
* Can we communicate in English and French, or would you prefer one language over the other?
The best way to say it depends on the situation and your relationship with the person you're talking to.