I use three 24" monitors for my external view, and a 22" for my instruments.
When I first tried opening different planes' 2D panels to drag over to the 22" monitor I found that some were stretched across all the monitors or, worse, didn't appear at all and caused the FSX cursor to do its spinny thing.
There is a simple fix for this.
Go to the main FSX folder and into SimObjects, then Aircraft.
Find the folder for the plane you want to edit and go into its Panel folder.
Take a copy of the panel.cfg file (naming it panel-orig.cfg or similar) and open the panel.cfg file in Notepad.
Find the panel you want to fix (for example [Window00]), and edit the following line:
Change it to:
If the line isn't there just add it.
This will reduce the width of the window to a third of the display (ie. one monitor if you're using three).
Save the file and fire up FSX to check that your panel is now displaying correctly. If you experience any problems you can just restore the original panel.cfg file.
You may also find that the external view is squashed up into the top part of the screen.
To fix this go back into the panel.cfg file for your aircraft, and look for the following section, usually right at the bottom:
Change the SIZE_Y to this:
Note that these fixes may not work for every plane. For example I have found big problems with the default Beaver that I have yet to overcome. I suggest you try one plane at a time, and if you experience problems just restore that plane's cfg, check the problem has gone, then try a different aircraft.
To avoid having to undock and drag your panel every time you want to fly, you can save the flights with the panel in the correct position on your second monitor. The steps are detailed in this post.
There is a lot more information on editing cfg files here.