Making music together requires not only individual skills and musical expertise, but the ability to coordinate one's actions with others. Typically, musicians try to improve their joint performances by practicing together. However, musicians encounter two kinds of obstacles when trying to improve their joint music making skills. One obstacle is the lack of sufficient opportunities for joint rehearsal. The other obstacle is the lack of research-guided exercises that would allow for learning transfer across pieces, partners, and different coordination problems. Based on the results of an ERC Consolidator project investigating the behavioral, cognitive, and neural mechanisms involved in joint action learning, we propose to develop an app that allows musicians at any level of expertise to improve their joint playing skills. Two aspects of the app make it possible to overcome the typical limitations of joint practice: First, with the knowledge gathered during the ERC Consolidator project we will be able to program virtual partners with whom musicians can train their ensemble skills individually outside of resource intensive joint rehearsals. Second, the app will focus on training the ability to flexibly shift between self-other integration and self-other segregation, which is a high-level skill with a large potential to generalize to a wide range of pieces and playing styles. The app will be web-based and freely accessible to a large pool of users. Its effectiveness will be thoroughly tested in the lab and in the field to assess its value for musicians playing in smaller and larger ensembles. As the first app directly training joint music making, TAPTAPP will provide new opportunities for musicians and has the potential to transform music education.