I don’t know but I guess it wouldn’t make much sense to have 2 sim cards if you couldn’t recieve calls and texts with both.
but you could simply give it a try: borrow a second sim card from a friend, put it in your fairphone and see if you can recieve calls from both numbers.
I don’t know of a french fairphone forum, but you can post topics in french here. there are many french users on this forum.
maybe this thread could interest you: multilingual forum