This preprint presents a web app (essentially a web page-based program) with which two or more users (peers) can view and handle 3D molecular structures in a concurrent, interactive way through their web browsers. This means they can share orientation and zoom level, commands and other operations in almost real time over the Internet through standard web pages. This web app, open source and built with the open source components JSmol for molecular visualization and Peer.js for WebRTC connection, provides a practical tool for online collaboration and teaching at a distance. More broadly, it illustrates the strong integrability of technologies for client-side web programming, and paves the way for similar apps for concurrent work in other disciplines. Web app is available at: http://lucianoabriata.altervista.org/jsinscience/concurrent-jsmol/concurrent-jsmol-visualization.html