User`s guide

Starting the X Window System
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You must also specify either:
Option "SecondMonitorHorizSync" "<hsync range(s)>"
Option "SecondMonitorVertRefresh" "<vrefresh range(s)>"
or:
Option "HorizSync" "<hsync range(s)>"
Option "VertRefresh" "<vrefresh range(s)>"
You may also use any of the following options, though they are not required:
Option "TwinViewOrientation" "<relationship of head 1 to head 0>"
Option "ConnectedMonitor" "<list of connected display devices>"
TwinView Edge-matched Display
The TwinView entries to obtain an edge-matched display (refer to Figure 3-3) with two
channels of 1920x1200 video are:
Option "TwinView" "true" # turn on dual channel mode
Option "MetaModes" "1920x1200"
Option "TwinViewOrientation" "RightOf"
Figure 3-3 TwinView Edge-Matched Display
Channel 0 Channel 1