Notice that the suggestion posted will only work with the proprietary "fglrx" driver. Users of the FOSS "radeon" driver (such as myself) should stick to xrandr.

Hi all.
My KDE 4.6.1 on Gentoo (nvidia) has started on second monitor), but on second kwin always crash at startup and I have to rerun it automatically. Also Alt+F2 input always shown on first display, not I'm using at the moment.

Also I'm trying to open just downloaded office document from Chrome in LibreOffice and office open's at first screen. Very unusable!

you might try adding (kwin --replace) to the autostart in system settings..
or install compiz and emerald and add (compiz --replace) and (emerald) to start up. hope this helps.

Greetings, I know this will sound absurd, but after trying everything in terminal and system settings, after a trivial monitor swap between two adjacent machines. I was endlessly banging my head against the wall (actually literally accidentally nailed my head on a part of my cube while reaching awkwardly, fitting, lol), I simply power cycled the monitor and it worked perfectly. The one caveat here is with an existing KDE instance the dual monitors had worked prior with different hardware. That said, sometimes the simplest solution is the easiest as that and one futile reboot prior were the only system changes I made. I really hope this actually works for someone else.