View a VNC server’s additional displays
If a client computer is running non–Apple VNC software and has multiple displays but you can’t see them when you observe the computer, you may need to add a custom port number for each additional display.
To control the default display on a VNC server that is listening on TCP port 5900, you set the screen sharing port to 5900. To control the second and third displays, you can set the screen sharing port to 5901 and 5902, respectively.
Add a custom port number to the computer info, as described in Set up a computer running VNC software.
Use the display number for the last number in the screen sharing port designation (display designations start at 0 for the default primary display).