It's not. I've managed to do it in xrandr. I have my main screen (and thus all screens) rendered at 200% and then scale down the other 2 back down to 100. Basically how macOS does fractional scaling.
It's not real multi-DPI no but effectively it does work. Does require a pretty decent GPU to render all screens at 200% before it scales them down though.
At one point I was using two 24" screens, one 1920x1080, one 3840x2160. I've tried messing around with settings, until I ended up on xrandr scaling as being, basically, the only solution. Five minutes later, the low-dpi display was in the closet, because I couldn't stand the blurry fonts.
It's ok for me but I used macOS before which also uses similar font anti-aliasing (macOS tries to preserve the look of the font in contrast to Windows which tries to align to pixel boundaries).
But indeed YMMV here.. There is no way to use non-anti aliased fonts at small sizes this way. For me it is fine, I would set it up the same way anyway but I forgot it's not for everyone.
