To my knowledge, I _never ever_ changed the xmonad compilation options
on my system. I'll give it a try, though.
It's less the xmonad configuration options than those of the Haskell X11 bindings. Also you must make sure the system Xinerama development libraries are installed when you rebuild the X11 bindings, or they will default to not supporting multihead.
(A common source of confusion here is that, while the Xinerama extension is no longer used, some of its lower level routines are still used by XRandR; in particular the routines for finding out what heads exist and where they are mapped on the virtual root.)