sorry, looking back I didn’t mention that

listen EADDRINUSE:  address already in use  128.0.0.1:8080

cd ~/MagicMirror1
npm start

in another window

cd ~/MagicMirror2
npm start

right

~/MagicMirror
and
~/MagicMirror2

assuming there is a coding error someplace, rename like this

~/MagicMirror1
and
~/MagicMirror2

then there is no, MagicMirror folder to use from

u cannot start it twice from the same folder

var config = {
  electronOptions: { x: 1920}, 

are you using docker or MagicMirrorOS

var config={
   electronOptions:{ x: 1920 },
   ...
    modules :[],
}

you can fiddle with the display options

var config = { 
     electronOptions : { ????? },
     modules : []
}

x and y describe the top left corner of the display
width and height describe the size and shape of the page

kiosk mode turns on fullscreen, no title bar, and some other things, not resizable I think… really a pain on mac)

the defintion of these options are here

https://www.electronjs.org/docs/api/browser-window

I use the electronOptions to have MM run on the second display on my desktop system

electronOptions: { x: 1920 },

so I tried

 electronOptions: { x: 0, width: 1920, height: 2160, minHeight: 2160, kiosk: false, fullscreen: false,  backgroundColor: "#00000001", autoHideMenuBar :true},

and it starts fullscreen on the 1st monitor (had to crop the screen shot to get it under 2 meg)

and

electronOptions: { x: 1920, width: 1920, height: 2160, minHeight: 2160, kiosk: false, fullscreen: false,  backgroundColor: "#00000001", autoHideMenuBar :true}, 

and it starts large, on the second monitor, but not quite full screen as the taskbar is not present on monitor 2.

neither the system , nor MM know about the physical placement of the displays