The data controller is a central player in data protection regulation. The data controller is the one who determines the purposes and means of data processing and makes substantive decisions about the data processing activities.
However, the data controller can not only act independently of a given data processing, but it may be that more data controllers jointly make decisions regardinf the data processing. This is also expressly clear in the definition of controllers set out in the GDPR, which provides that the data controller is the natural or legal person, public authority, agency or other body which, alone or jointly with others, determines the purposes and means of the processing of personal data. (This is, of course, not a novelty, current data processing rules contains similar definitions, so it is possible that multiple data controllers jointly define the purpose of data processing.) On the other side, GDPR makes the situation quite clear when it states that if the purposes and tools of data processing are jointly determined by two or more controllers, they shall be joint controllers (see Article 26 of the Regulation).