Is it permissible for us to obtain French citizenship while retaining Tunisian citizenship?

It is not permissible to obtain the citizenship of a non-Muslim country except out of necessity, such as a Muslim facing hardship in their own country without finding a Muslim refuge, or for an overriding legitimate interest, like da'wah (inviting to Islam), while openly practicing the rituals of the religion. If this need or interest does not exist, then the original ruling is impermissibility. Similarly, residing in non-Muslim countries is not permissible except under certain conditions and regulations.