1. Where two or more controllers jointly determine the purposes and means of processing, they shall be joint controllers. They shall in a transparent manner determine their respective responsibilities for compliance with the obligations under this Regulation, in particular as regards the exercising of the rights of the data subject and their respective duties to provide the information referred to in Articles 13 and 14, by means of an arrangement between them unless, and in so far as, the respective responsibilities of the controllers are determined by Union or Member State law to which the controllers are subject. The arrangement may designate a contact point for data subjects.
Article 13 GDPR. Information to be provided where personal data are collected from the data subject
Article 14 GDPR. Information to be provided where personal data have not been obtained from the data subject
2. The arrangement referred to in paragraph 1 shall duly reflect the respective roles and relationships of the joint controllers vis-à-vis the data subjects. The essence of the arrangement shall be made available to the data subject.
Separately, the issue of joint controllers is also related to making data subjects aware of the risks, rules and safeguards. Article 26.1 requires joint controllers to determine their respective responsibilities for complying with obligations under the GDPR in a transparent manner, in particular with regard to the exercise by data subjects of their rights and the duties to provide the information under Articles 13 and 14. Article 26.2 requires that the essence of the arrangement between the data controllers must be made available to the data subject. In other words, it must be completely clear to a data a subject as to which data controller he or she can approach where they intend to exercise one or more of their rights under the GDPR.
 Under Article 26.3, irrespective of the terms of the arrangement between joint data controllers under Article 26.1, a data subject may exercise his or her rights under the GDPR in respect of and against each of the joint data controllers.
3. Irrespective of the terms of the arrangement referred to in paragraph 1, the data subject may exercise his or her rights under this Regulation in respect of and against each of the controllers.
General Data Protection Regulation (EU GDPR)
The latest consolidated version of the Regulation with corrections by Corrigendum, OJ L 127, 23.5.2018, p. 2 ((EU) 2016/679). Source: EUR-lex.
(79) The protection of the rights and freedoms of data subjects as well as the responsibility and liability of controllers and processors, also in relation to the monitoring by and measures of supervisory authorities, requires a clear allocation of the responsibilities under this Regulation, including where a controller determines the purposes and means of the processing jointly with other controllers or where a processing operation is carried out on behalf of a controller.
The existence of joint responsibility does not necessarily imply equal responsibility of the various operators involved in the processing of personal data. On the contrary, those operators may be involved at different stages of that processing of personal data and to different degrees, so that the level of responsibility of each of them must be assessed with regard to all the relevant circumstances of the particular case. Actual access to personal data is not a prerequisite for joint responsibility (p. 68-72).