I'd go with a live image of whatever distro you're most familiar with. I usually go with the arch iso or endeavor os if I need a GUI.
You can't set 2 different DPIs for the monitors on X11. On one monitor everything is just going to be bigger than the other. Depending on the DPI difference it can be basically unusable.
If your monitors are different DPIs then multimonitor X11 is awful.
If you're questioning why anyone would have monitors with different DPIs remember that laptops exist.
I'm aware plasma has it but I don't like plasma that much. I'm currently waiting for it to be integrated into other compositors (mainly sway).
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