Modern systems exhibit unprecedented complexity due to their increased scale, interconnectedness, and the heterogeneity of their digital and physical components. In response to scaling challenges, the system-of-systems (SoS) paradigm proposes flexible aggregations of subsystems into a larger whole, while maintaining the independence of subsystems to various degrees. In response to the cyber-physical convergence, the digital twin (DT) paradigm proposes a tight coupling between digital and physical components through computational reflection and precise control. As these two paradigms address distinct parts of the overall challenge, combining the two promises more comprehensive methods to engineer what we call systems of twinned systems (SoTS). The noticeably growing body of knowledge on SoTS calls for a review of the state of the art. In this work, we report on our systematic literature survey of SoTS. We screened over 2500 potential studies, of which we included 80 and investigated them in detail. To converge SoS and DT, we derive a classification framework for SoTS that is backward compatible with the currently accepted theories of SoS and DT.