It is a pretty long running issue in sddm since version 0.20
There are a few ideas at the issue link how to configure /usr/share/sddm/scripts/Xsetup
Requires this line to be added into /etc/sddm.conf to work:
DisplayCommand=/usr/share/sddm/scripts/Xsetup
Be sure to get your monitor names with this command while being in a X11 session:
xrandr | grep -w connected
(I know you use X11, but I wanted to state that just in case somebody that normally uses Wayland also has this multi-monitor sddm issue and sees this thread. Monitor names are different across session types and the script uses X)