Joint PhDs or co-tutelles pre-suppose the writing of a co-tutelle agreement, which determines the practical modalities of the thesis: duration, follow-up, presence of the student in the universities, defense, language (of the writing, the defense), etc.
These methods vary from one university to another: in Anglo-Saxon countries, for example, the annual registration fees are high at the PhD level, and the co-tutelage agreement may stipulate that the doctoral student must be present for six months in the partnering university.
The Campus France website specifies the principles of co-tutelles. If it is a co-tutelle agreement with a university that is already a partner of Rennes 2 University, the terms of co-supervision will be easier to establish. In all cases, the thesis supervisor at the foreign university must contact your director at Rennes 2 to agree on the terms of co-supervision.