Every time I boot up, KDE appears to forget that I want my monitors to be positioned left-to-right and instead defaults to mirrored config. But, after a lot of cursing and a little googling, I found an answer so it'll no so much keep your settings, but reset its broken config to your settings.
1. Hit ALT-F2, then enter "display" to run the Display Settings app.
2. Configure your settings as you'd like. Note that if the Apply button isn't active after your changes, you can change/revert something like a Position: button to make it active.
3. On restart, KDE may forget your dual-monitor settings. So, to prevent this, go look in your ~/.kde/share/config/krandrrc file:
[Display] ApplyOnStartup=true StartupCommands=xrandr --output "DVI-I-1" --pos 1920x0 --mode 1920x1200 --refresh 59.9502\nxrandr --output "HDMI-1" --pos 0x130 --mode 1920x1080 --refresh 60\nxrandr --noprimary
4. Copy the configuration into a new file, and replace \n with newlines. I like to put scripts like this in /etc/X11 because they relate to screen res and positioning.
# from ~/.kde/share/config/krandrrc xrandr --output "DVI-I-1" --pos 1920x0 --mode 1920x1200 --refresh 59.9502 xrandr --output "HDMI-1" --pos 0x130 --mode 1920x1080 --refresh 60 xrandr --noprimary
5. Ensure the script is readable/executable for all users:
chmod 755 /etc/X11/1920x2.sh
7. Click Add script... and browse for the script you created above.
8. Reboot and watch the magic unfold.