Can I create 2 "displays" on a headless server? I have 2 Mac Minis.  One is to be used as a headless server while the other is a general-purpose user desktop machine.  The user system has two monitors.  I have set up the server to allow screen sharing, which works fine to show its one display on one user monitor.

The question is whether there is a way to tell the server that it should pretend to have two displays (instead of pretending to have one) and allow those to show on corresponding user monitors.
No, only one screen can be viewed at a time over Screen Sharing. However, if you have configured two screens, you can view each screen, one at a time, by selecting "Displays" from the "View" menu item in the screen sharing app on your User Mac.  This way you can switch which screen to view.