Of course you can interact with two monitors at once. I do it every day. My current standing desk setup is an MBPR15 on top of a wine case on my desk, with a V

Of course you can interact with two monitors at once. I do it every day. My current standing desk setup is an MBPR15 on top of a wine case on my desk, with a ViewSonic 24" 4K display to the left, hanging in portrait mode on the Amazon Basics version of an Ergotron arm. Best dev rig I've ever had! Quite often I'll put docs and references on the external monitor while I write code in Visual Studio or IntelliJ on the Mac's display. This works especially well with the monitor in portrait mode. Then I leave the mouse cursor over on the external display and use two-finger scrolling there, while using the keyboard in the editor window. If this isn't interacting with two monitors at once, I don't know what is. And it's a huge improvement over a single monitor.