In this article, I bring together three recent ideas from political theory and constitutional law. The first is the notion developed by Jürgen Habermas and Markus Patberg of a pouvoir constituant mixte. Complex polities such as supra-state federations should be understood as constitutional entities that combine two or more authorizing subjects. The default “mixed” conception is that of dual sovereignty, split between the federations’ member collectives and member individuals. The second idea is that of Stephen Tierney, that sub-state national societies should be seen as involved in struggles over constituent power. The third is the idea and practice of sovereignty referendums, which I illustrate by looking at the 2017 Catalonia independence referendum. I contend that, at a minimum, sovereignty referendums, especially lost ones, should be understood as strategies to bring about, shift or complement constituent powers, in achieving compound or mixed sovereignty. Finally, I venture into some ideas about the criteria of legitimacy for such endeavors, in distinguishing the activation of constituent powers from their articulation and exercise.