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OK, all or any of the following points may be totally wrong, but from what I undertand that's how it works:

You just install the second copy into a different folder.  You don't need a second licence, because it's on the same machine.
As far as I understand it, the advantage comes from the fact that Windows will allocate different cores of the CPU to both processes, i.e. they really do run in parallel, and they use different graphics cards too.

You make one the main and the other an extra visual to the first.  The main has to be launched first, and you wait until you'r "in the cockpit".  Obviously you assign each monitor to only one of the processes.

One point which was important: the main X-Plane process will use the graphics card that is connected to the monitor that is also the main (start-up) monitor for Windows. This can be set in the Windows displays panel.  So make sure that (1) you know which is your best graphics card, and set Windows to use the monitor(s) connecte to that one as the main Windows display(s). (2) connect the rest to the other card.  Windows will give  the graphics card to which Windows's main display is connected to the main X-Plane process.  Then the other process, started later, will be given other CPU cores and since it will be set up to use the other monitors, it will necessarily get the other card.

When you first start your machine, Windows will use all three monitors, and it is entirely up to you to set which one is the "main" one (where the login window will appear).  That setting, which is in Windows, then decides which card is your "main" graphics card.  That in turn decides which card will be used by the main X-Plane process.

Now, note that I carefully avoided the word "instance".  Because:  an instance is a process that runs on the same code in the same place in memory as another process does.
Here we are talking about having two copies of the program code each in different places in memory, each having one process running them.  So you need enough memory to avoid too much swapping.  I have no idea how one would load only one copy of the program code and run two processes on it, in Windows.  In industrial control systems it is common to run several instances on the same code in memory, but then there is usually no swapping etc.  That's not how PC operating systems are designed.
In my case I have three full and identical copies of X-Plane + data, in three different folders on disc.  Windows loads three programs, gives them CPU cores, and has no clue that those programs are actually identical.

Hope this helps.
Feb 5, 2018 by robertcailliau
Hi, I've seen you have a lot of experience with Xplane and I was wondering if you can help me. Many years ago I was an FSX  fan and builded a PC (it wasn´t that bad by that time  - 2013), it has a core i7 3770K, 16GB ram and 128GB SSD and a GTX 660. Now I decided to go back into simulators and bought Xplane 11 because it's an updated product. I'm running three monitors:  2 are 16:9 aspect and one 4:2 aspect for the middle, in an attempt to get better frame rate (15 -18fps right now) I've decided to get a second GTX 660 card to implement an SLI configuration without knowing Xplane doesn't benefit from SLI (du-hu, I've should read about it before buying the second carf). My questions are: I've read in other posts that you have multiple instances of Xplane 11 running in the same computer using different GPUs, how can I do that? is there a special procedure to install Xplane twice? do I need to buy another license for doing that?  Do your think I can get benefits (with fps) by doing that with my second card?  Right now I'm using only one card with the 3 monitors (not 1080p, they are old Dell 17" LCD monitors) connected independently (not nvidia surround) and I'm getting 15-18 fps with regular quality. I'm not planning going 1080p right now but would love to get better fps. I will really appreciate your advise.

Feb 5, 2018 by jchinchilla

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