On Linux/X11 in regular display mode, a window can’t span multiple X-Screens, and windows involved in PTB’s mirroring modes must also be located on the same X-Screen. So your specific xorg config for three separate X-Screens won’t work for this purpose, only for having two totally separate stimulus windows on screen 1 and 2, e.g., for stimulating two subjects in parallel or similar, and the desktop GUI with Matlab/Octave on screen 0 (or a 3rd independent stimulus window). There are different solutions however, depending on the specific needs and hardware setup - mirroring an image is something that sounds simple, but is quite involved, depending on setup and needs, if low-level control / accuracy / timing is of any importance. The next upcoming Psychtoolbox 3.0.19.0 release will have a few new tricks useful for display mirroring, at least applicable to graphics cards with fully open-source drivers, ie. non-NVidia.
Given this question is kind of off-topic for the title of this discussion thread, please post all followup answers under the following fitting topic to continue the discussion:
My first two questions though would be:
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Why not use the dual-link DVI “console monitor” output of your ViewPixx for a simple hardware solution? Assuming you really just need a mirror image of the stimulus, nothing fancy, and can connect a suitable secondary monitor without compromising timing?
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If 1 is not an option, do you use Ubuntu 22.04-LTS or later? What is the resolution of your “console monitor” for the experimenter? It provides a new trick wrt. display mirroring, specifically contributed by myself to X-Server 21 for such scenarios, at least for graphics cards with fully open-source graphics/display drivers, ie. non-NVidia.
→ Please answer in the linked topic above, not here.