if I have dock window (xfce4-pannel) already running before switching to the 2 screen setup (via xrandr calls), then it still docks (awesome adjusts the 'usable' area of the screen to exclude the edge) nicely not only to the left edge of the 1st screen, but also to the right side. If I kill xfce4-pannel and start it again already in 2 screen mode, I can only dock it to the left edge of the 1st screen or right edge of the 2nd screen (so awesome looses a notion that there are dockable edges on the border between two screens)