Would be a bit annoying to use, as the mouse pointer is only active on the desktop mode screen, but I imagine you could mirror the phone screen onto a displaylink display or adaptor with the Displaylink Presenter app while also using desktop mode on another monitor.
Would have to use your finger on the phone screen, rather than the mouse to interact with any apps there. But an external keyboard should work with apps on both displays. Could keep an app, or with split screen two, that you mainly monitor, rather than interact with, up on the displaylink/phone screens.