You need to be running an instance of the game for each monitor you want to power, assuming its a view of the plane or outside the plane. One instance of the game will allow you to have the instructor view or a 2d panel dragged aside on the second monitor. You cannot however, have three views (left, forward, and right) in 3d cockpit mode using one instance of the game.
This is straight from the X-Plane configuration instructions....
Configuring a Multi-Monitor Simulator
There are two general ways of configuring multiple monitors. You can either have your monitors all connected to one computer, running one copy of X-Plane, or you can have multiple different computers all networked together, each one with its own monitor and its own copy of X-Plane.
In general, using multiple displays on one computer will be more restrictive regarding the ways in which you can configure the simulator. Networking many computers together will be more flexible, but it will also be much more expensive.
Driving Multiple Displays from One Computer
If your are using only one copy of X‑Plane on one computer with several monitors, you will be able to configure the view as
- one large display of one view or
- one view and one Instructor Operating Station.
X-Plane only supports one type of view (3-D cockpit, forward with no scenery, etc) at a time per copy of X-Plane.
If your multiple monitors are configured as a single large display in your operating system, all you need to do to have X‑Plane fill the screen with a single large window is go to Settings > Graphics > Monitor Configuration and change the drop down to “Full Screen Simulator.” If X‑Plane is still only filling one screen, you may need to pick “Custom” from the Resolution drop down, and change it to the combined size of all your monitors (for example, two 1920x1080 monitors should be entered as a custom resolution of 3840x1080). You will most likely want to adjust the “Lateral field of view” as well.
If, on the other hand, your monitors are configured in the operating system as separate displays, the simplest option is to have a regular, windowed version of X‑Plane which you manually resize to fill as much of your display as possible.
If your monitors are separate displays and you also don’t want to have X‑Plane windowed, in the Monitor Configuration box you will need to set each monitor to “Full Screen Simulator.” Then you will need to adjust the offsets in the Visual Offsets section.
For a wrap-around, 3-monitor configuration:
- Left monitor: set the Lateral rotational offset to the negative of your main monitor’s Lateral field of view (FOV)
- Center monitor: no offset
- Right monitor: set the Lateral rotational offset equal to your main monitor’s (positive) Lateral FOV
For a flat, 3-monitor configuration:
- Check the box to Enable flat (fractional) offsets
- Enter –2 for the left monitor’s offset
- The center monitor offset should be left at 0
- Enter +2 for the right monitor offset