You should be able to pull this off using borderless window mode and opening multiple windows with their own GameViewportClients in them. This isn’t something we really test very often, but you’ll probably notice that the Unreal Editor itself supports any combination of monitors, and it is presented using the same rendering system as a game uses. You may need to write a bit of custom code, but there should be no inherent technical limitation that would prevent you from setting up your product this way.
–Mike