This script will add the lock icon image to your wallpaper to represent screen is locked, it will then copy the completed image and join it together making 1 long image displayed accross both screens. this assumes your monitors are of the same resolution.
create a symlink for the lock as slock in /usr/bin
sudo ln -s /path/to/lock /usr/bin/slock
now you can use slock, in my example i use xautolock
xautolock -locker slock -time 4 -corners -+00 -cornerdelay 5