Hi
At first.... This thematic is not Mobiflight related.... Cause MF support only 7Seg and small 4 Line LCD Displays. So the whole thematic need a "external" solution.
Basicly we work with 3 Systems here....
1. As Stephan explaned.... The easy "Outsourced" Window Mode.
Lots of AddOns allow to unlock a 2D Panel ( e.g. the ND, PFD, FMC, ECAM and so on) Then the pure Display is a simple Window you can Move on your screen.
And with a second screen ( So the Display is connected like a screen via VGA/HDMI/DVI/DP/US
you simply "enlarge" the Desktop over multiple screens and you move the window to the other Display.
Disadvantage: This reduce FPS.... We got reports that some systems slow down to real hard delays and stuttering Pictures with FPS below 20. More critical as much displays you use.
2. Windows that are "self running" .
High Advanced AddIns like ProSim, Magenta or Jeehell allow to use self running programms for each Display.... Mean you got a "server" App on the Flight Sim Computer..... This support the needed Data. Each Display is like a own Programm that recive Data from the Server and simply create a Picture of the Display ( Window mode)
These are resizeable and moveable.... So you can set it to the dimension of your Screen and put it there.
Technical like Point 1.... But with the big benefit it is self running and it can be used in a Network.
So FPS not go down in case the Displays run on a second Computer.
( High End Cockpits mostly worrk with 4-6 Computers... Main PC got Sim and Outside View.... PC2 got PFD/ND/FMC Cpt. .. PC3 Got FO side.... PC4 Got ECAM and so on)
3. Individual Solution.
Here the Display is not a screen.... It is a Element of a Controller ( E.g. a Arduino Mega).
This means you got a sketch/firmware on it that controll the Display..... And you need a Software on Flightsim Computer ( and maybe a client) that get the needed Data from Sim and send it to the Mega.
Here for example PMDG is fine. It support the whole FMC Data via FSUIPC. So in theory with programming skills you could build this !
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Summary:
Way 1 for "normal" Cockpits.... Way 2 for advanced Sims with Multi PC Setup .... Way 3 is the perfect solution but need adfvanced programming skills.