I used xrand to generate xrandr --output LVDS-1 --mode 1366x768 --pos 1920x0 --rotate normal --output HDMI-1 --off --output VGA-1 --mode 1920x1080 --pos 0x0 --rotate normal
command.
xrandr proposed saving it in ~/.screenlayout/ folder, and it is not automatically used on logging in.
Running it from terminal mostly works, but display of workspaces in taskbar is horizontally squashed.
Adding it to autostart (via ~/.config/lxsession/Lubuntu/autostart) has no effect whatsoever.