The Chromium-only Presentation API opens a second page on an external display and messages it. From a click, call new PresentationRequest(['receiver.html']).start() to pick a screen, then connection.send(...); the receiver listens on navigator.presentation.receiver.connectionList. request.getAvailability() decides whether to show a cast button (headless Chrome 1 reports false). For two desktop monitors, the Window Management API (Screen Display) is often simpler.