OpenELEC is configured to display only on the primary display, so you need to change the configuration to enable additional displays.
A good example is if you're running OpenELEC on a laptop, you may want to plug in your TV via one of the external ports - by default OpenELEC will only appear on the primary (internal) display.
Things you'll need:
Open an SSH session to your OpenELEC machine then at the prompt, type:
cp /etc/X11/xorg-nvidia.conf /storage/.config/xorg.conf
This copies the currently running settings for your graphics card and moves them to the configuration files directory. Now that we've got it somewhere we can make changes to it, we can edit the configuration file:
Scroll down until you find the following text:
Section "Device" Identifier "nvidia" Driver "nvidia"
Underneath, add the following text:
Option "TwinView" "on" Option "TwinViewOrientation" "clone"
X to exit. You'll be prompted to save the file - just hit
ENTER to save the file.
You should be back at the prompt now, so just type exit to close the prompt with
Reboot your OpenELEC box and now the second screen should work as clone of the primary laptop monitor.