hiI've tried it. I set the screen in portrait in windows 10 screen parameters. I didn't tried it with our planetarium with duplicate screen. I have to try.
I'll tell you, if this intersting you.
Yes, obviously you need to set your operating system and/or graphics driver to use portrait mode, but I was talking about this setting in Space Engine:
If it doesn't look right, and you don't have your particular resolution available under "Custom Display Resolution", click on Windowed (Apply), edit the resolution numbers if necessary (click Apply again). Then you can click on Fullscreen and Apply again. Should be good to go.