Moving the game is just to isolate it from Steam and updates you may not want and possibly overwriting any settings made..
The IL21946 conf.ini file needs this setting for multiple monitors
iirc
[window]
width=1920
height=1080
ColourBits=32
DepthBits=24
StencilBits=8
ChangeScreenRes=1
FullScreen=1
DrawIfNotFocused=0
EnableResize=0
EnableClose=1
SaveAspect=0
Use3Renders=1
Usually 3 monitors are used with Matrox triple head to go or something similar.
But newer GPU hardware can be setup in the Nvidia control panel,
it's been a while since i got involved with multiple monitors in IL2, you may have to explore Google
