/etc/X11/xorg.conf Example (1920x1080, use cvt 1920 1080 to get correct modeline values
Section "Monitor"
Identifier "Monitor0"
Modeline "1920x1080_60.00" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync
VendorName "Monitor Vendor"
ModelName "Monitor Model"
EndSection
Section "Monitor"
Identifier "Monitor1"
Modeline "1920x1080_60.00" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync
VendorName "Monitor Vendor"
ModelName "Monitor Model"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1920x1080"
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 4
Modes "1920x1080"
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 8
Modes "1920x1080"
EndSubSection
EndSection
Section "Screen"
Identifier "Screen1"
Device "Card1"
Monitor "Monitor1"
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1920x1080"
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 4
Modes "1920x1080"
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 8
Modes "1920x1080"
EndSubSection
EndSection