For some reason the server is using legacy multihead mode, which as you've observed is not all that friendly.  (BTW, in legacy multihead there is *no* way to move windows between screens.  This is an X server limitation; xmonad has no control over it.  It's one of the reasons that there exist hacks such as xmove2.)  I would have expected that there be some way to tie them together with xrandr, instead of it using legacy mode; but on the other hand I seem to recall some vendors' video drivers don't support that part of the xrandr spec.