What display manager/window manager can support multiple monitors with different dpi?

IIRC neither gnome nor kde currently support dpi scaling on wayland, so the answer is currently none.

Update: Work has started on multi-DPI in Gnome 3, no indication yet on whether it would be usable in 3.22...

When calling xrandr, you can add --scale 2x2 for the non-HiDPI output. This will render the display image in high resolution, but scale it down before displaying it on the monitor. It's doesn't always look pretty, but it's usable. For example, I'm extending my internal 2560x1600 retina display eDP-1 with an external non-HiDPI monitor DP-1:

#!/bin/sh
EXT=`xrandr --current | sed 's/^\(.*\) connected.*$/\1/p;d' | grep -v ^eDP`
xrandr --output eDP-1 --auto --pos $(((3840-2560)/2))x2400 --scale 1x1 \
       --output "$EXT" --auto --scale 2x2 --pos 0x0

See also this gist.