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.
1. Go to Menu -> Preferences -> Arandr
Another rectangular box labeled HDM-1 overlayed the original one should show up. You can then drag around the boxes to the desired positions.
Hope this kinda help! 😀