@sciss You can use an HDMI -> SCART converter like this: https://www.pollin.de/p/hdmi-konverter-smart-hdmi2scart-hdmi-zu-scart-600457
But the other question is if the I/O system on the little Pi can push frames fast enough to keep two channels running at full speed? The Pi4 should be able to but I don't know about the others. You will definitely need a really good SD-Card.
@sciss re dividing single HDMI output into multple outputs / tiles, something like Matrox Triple Head might do it, however I have no experience with it.
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