For many years I used nvidia drivers and utilities along with an
xorg.conf to set up 2 to 3 monitors on KDE and Xfce. Recently I
switched to the new AMD NAVI GPUs (RX 590 and RX 5700 XT) and the new
Slackware-current amdgpu driver stack. In KDE, setting up dual monitors
was easy in their display GUI, but Xfce-4.12 was problematic. No matter
what I did, the second monitor was always on the virtual right side of
the primaty monitor, or the primary and secondary were switched. A
workaround is to invoke xrandr directly:

xrandr --output DisplayPort-0 --primary --mode 3840x2160 \
       --rate 60     --output DisplayPort-1 --mode 1920x1080 \
       --rate 60 --left-of DisplayPort-0

Placing this command in $HOME/.xinitrc automates the process. This also
fixed tthe monitor jumbling when using the screensaver and/or powering
off the monitors.