Lubuntu does not have multi-display settings by default (see source). The one suggested by the forum is using Arandr, which is available on Lubuntu Software Center.
The interface is a bit confusing to work with. (Keep forgetting how to do this every time after a fresh install..) So this is just a short tutorial on how this Arandr program works.
How to’s:
1. Go to Menu -> Preferences -> Arandr
2. On the Menu bar, click on Output -> HDMI1 -> Active
Another rectangular box labeled HDM-1 overlayed the original one should show up. You can then drag around the boxes to the desired positions.
3. To turn off the monitor, right click on the monitor box and de-select “Active”
4. Final step, MUST click on the check mark below ‘Layout’ after you got your desired display settings done.
Hope this kinda help! 😀
My Little Space 
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