mtk0 may have been referring specifically to the case of multiple X server display screens (i.e. :0.0 , :0.1, .... ) as opposed to the operation of multiple monitors under/constituting one X server display screen.
Originally Posted by mtk0
That is something I can attest too, though I suspect that it is due more to xrandr then it is the WM/DE.
Example: everything works perfectly when using an tri-headed eyefinity card, driven by the radeon driver. Add a second graphics adapter into the mix and you quickly run into headaches -- the 3 physical monitors on the eyefinity card (that constitue screen 0) are merged into one giant monitor as far as the WM/DE is concerned -- the result is that the taskbar is stretched across all of them; window maximization is no longer per monitor but rather across all 3; notifications and non-fullscreen windows that are opened end up appearing anywhere across the 3 monitors. Same thing occurs on the monitors (constituting screen 1) being driven by the second card. Moving the mouse cursor from the leftmost monitor of screen 1 results in the curson ending up appearing on the right edge of the leftmost monitor of screen 0, as opposed to (as expected) its rightmost monitor's right edge .... (this behaviour is actually fairly bizarre given the way the monitors are treated by the WM/DE under this dual graphics adapter/x-server display dual screen scenario). etc....