I have reported this here also: https://bugzilla.xfce.org/show_bug.cgi?id=10511 I don't know if the bug is in the switcher or in the multimonitor support.
Sorry, the culprit was neither the switcher nor the display settings but missing configuration of lightdm. http://askubuntu.com/questions/73804/wrong-login-screen-resolution/331988#331988