[KevinH]

Sorry, can not help here as the paradigm on MacOS is to allow multiple MainWindows from the same instance of Sigil.

Do your two displays use a single graphics card/driver instance or do you have multiple graphics cards installed?

If sharing the one graphics card and mapping the two displays to different areas of the virtual display (side by side displays or top and bottom displays) it should work.

But In all cases the two instances will share Sigil settings files, so perhaps one is trying to open up on top of or behind the second instance?

Perhaps it is a problem with Qt WebEngine? Have you tried hiding Preview and then launching the second instance?

Any other error messages get printed when you launch one from the command line?

I will try this on Linux (two instances of Sigil running) but I only have a single display.