Digital twins are often thought of as cyber-physical systems that model the behaviour of physical processes and systems. Software engineers’ uniquely virtual engineering material software is, itself, “cyber”, leading to the possibility of cyber-cyber digital twins. A cyber-cyber digital twin is a deployed software model that executes in tandem with the system it simulates, contributing to and drawing from that system’s behaviour. Although cyber-cyber twins share many of the attributes of cyber-physical digital twins, their doubly virtual nature also creates unique open research problems and opportunities, with applications in measurement, prediction, validation and optimisation of twin behaviours. This talk will outline Facebook’s cyber-cyber digital twin, WW, a twin of Facebook’s WWW platform, built using Web-Enabled Simulation. The talk will also cover experience, issues and approaches when collaborations between academia and industry.