The video descriptions doesn't tell how he did that, but there are are number
of options to get Compiz working on more than two LCDs on nvidia hardware:
* Nvidia QuadroPlex with SLI Mosaic technology (up to 8 LCDs, 8192x8192).
Here, the GPUs are combined on driver level so that's you don't need
to use Xinerama. I didn't yet have seen any negative comments on
functionality, but those devices are pretty expensive (around $10.000).
Unfortunately, it isn't possible to apply SLI Mosaic to consumer cards.
* Xgl on top of Xinerama (no limits as far as I know)
Xgl is an X-Server that works on top of another X-Server. Xgl only
uses OpenGL as rendering back-end. This makes it very fast and
flexible. Unfortunately, Xgl is deprecated and not maintained any
more since 2008. Getting Xgl working with recent distros doesn't
seem to be an easy task. I think Xephyr with 3D support should make
a replacement for Xgl, but my understanding is that the 3D support is
still experimental. (never tried this myself).
* Splitting video outputs with Matrox TripleHead2Go (6 LCDs, 5040x2100)
Matrox offers a video splitter that i.e. takes a single 5040x1050 DVI signal
and splits it into 3x 1680x1050. With two of those boxes attached to the
two outputs of a single GPU video card, you can get up to 5040x2100
(2x 3x 1680x1050). I didn't noticed any problems with such a setup, but
the TH2Gs are fairly expensive (about $300 per box).
More details can be found, here: