This question seems off-topic for the thread you posted in, but anyway…
What @Ian-Max-Andolina says is an option. For Linux with graphics cards that have an open-source driver, e.g., AMD, Intel, there is another option though that’s worth trying, and that is using XOrgConfCreator + XOrgConfSelector to create and install a custom xorg.conf for this mirroring.
On a dual-display setup, you’d need to create a single X-Screen config with only screen 0, but in XOrgConfCreator select 'y’es for advanced settings and the answer 'y’es to the question “Use AsyncFlipSecondaries mode for multi-display setups …?”.
Once that is setup + XOrgConfSelector + logout + login, the X-Server will only vblank synchronize flips to the main monitor for properly timed and tear-free stimulus presentation, but the secondary monitor will tear/flicker on animations. By default the higher resolution monitor will be synced to be usable as stimulus monitor, the lower resolution monitor would have to be the desktop GUI / Matlab monitor and be the one with undefined presentation timing and tearing, but certainly good enough as a “experimenter control monitor”. If both displays have the same resolution, then the stimulus monitor needs to be manually set as “primary monitor” / “primary output” to break the tie, so the system knows that timing and quality matters on that monitor. Running PerceptualVBLSyncTest will tell you very quickly if the right monitor is properly synced.
This all should work then with either both monitors set to “mirror” or “clone” each other, or in extended desktop mode by use of PsychImaging()'s “software mirroring” task.
Cfe. these two old discussion threads for more bread crumbs:
and