OK - I worked it out - it's actually REALLY easy.
You need to run 1 instance of XP11 to generate the simulation (the MASTER), and one instance to generate each external view (the Slave/s).
A. Set up the FIRST instance of XP11 as MASTER
1. Launch XP11
2. In Settings/Network, set 'This machine's role' = Master
3. for MACHINE WHOSE VIEW IS INDEPENDENT FROM OURS
- enter the SAME IP address as the Master (THIS MACHINE'S IP ADDRESS)
- set the UDP port to 49020
4. Leave the UDP ports at the default values :-
Port we receive on = 49000
Port we send from = 49001
5. Click DONE
6. Start your flight, then PAUSE
B. Set up the SECOND instance of XP11 as SLAVE
1. Launch the second instance of XP11
open terminal and type "open -n -a [pathname]", where [pathname] is the path for your XP11 app.
It's actually easier to type "open -n -a ", then drag the X11 app onto the end of the text.
2. When the app launches, immediately click the 'Settings' button and go to Networks.
3. In Settings/Network, set 'This machine's role' = External Visuals
4. For MASTER MACHINE, set the IP address to the same as the Master (THIS MACHINE'S IP ADDRESS)
Leave the UDP port at 49000
5. Set the UDP ports :-
Port we receive on = 49020
Port we send from = 49021
6. Click DONE
7. Click Resume Last Flight or New Flight - the MASTER will automatically set the aircraft and airport.
C. You will see the 2nd instance of XP11 is PAUSED, and the aircraft and airport are the same as the MASTER.
D. Set whichever views you like, then un-pause - and fly !
SOME ADDITIONAL TIPS
1. You can run as many instances of XP11 as you like - one for each view.
2. I had 3 instances running with different simulations - with virtually no degradation in frame rate.
3. You can use this little AppleScript to launch 1 instance of XP11 :-
do shell script "open -n -a [pathname]
A trick - if any segment of your pathname includes a space, then enclose in single quotes ... like this ...
do shell script "open -n -a /Applications/'X-Plane 11'/X-Plane.app"
You can use this as many times as you like - just run it once for each instance.
5. Once you've tested the script, save it as an 'Application' - then just double-click to launch a new instance of XP11.