Found the window manager splitwm[1].

  Interesting  idea.  We  then  found  out this can be done using a fake
  Xinerama library[2], so you can use any window manager you like. Neat!

  If I still had a widescreen monitor, I'd surely  give  these  tools  a
  try.

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  1. https://bbs.archlinux.org/viewtopic.php?id=157080

  2. http://home.kde.org/~seli/fakexinerama/