What's wrong with xmonad's handling of xinerama? I seem to recall both awsome[a] and scrotwm[s] have decent handling of mulitscreens setups for drivers that doesn't support xrandr, or used twinview -- maybe I mis-remember what used to be my problem -- but these days xmonad works nicely for me, whenever I need more than one screen.
It "works" fine, it's just completely unintuitive and user-hostile. Tried it for a week or two, even with the cheat sheet taped to my monitor I could not build an intuition for what it was going to do. I'm sure there's a reasonable-sounding explanation of how it worked (each monitor is a view of the stack-set, or some such thing) but trying to use it was intensely irritating and it never got better.
[a] http://awesome.naquadah.org/ [s] http://www.peereboom.us/scrotwm/html/scrotwm.html