Xinerama emulation
Alternative method
Xnest can also be used to bypass Xdmx altogether. Both previous steps can be replaced with one operation:
Xnest -geometry 1440x900 -scrns 2 -ac :3 &
However I found Xnest to be much less stable than Xdmx.
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intro | part 1: Create nested displays | part 2: Set up Xinerama | part 3: Alternative method