After setting up up a dual screen setup on Arch Linux inside of KDE, rebooting or logging off discards any changes made to the set up, and you go back to the default setting. In my case it went to both screens being clones. Even changing the setting inside of KDE's monitor settings , and saving it as the default setup didn't work.
To fix this, you have force it using the xorg.conf file to hold the dual screen set up.
To do this run the nvidia settings , by going to K-Launcher > type in nvidia , and then click on the nvidia settings shortcut.
Inside nvidia settings config the display to exactly what you want, then hit the "Save Configuration" button. At this point just save it to a location where you know it's location.
Simply replace your old xorg.conf with the new one that you just generated. This should fix this problem, and also take care of any resolution issues that may arise.