I was able to run Catalyst using 2 TH2G for 6 x 1024x768 on a Quatro FX. It was a bitch to setup and get working and I had to do it in stages but it worked.

It will not work to use 2 boxes at 1024x768 on an x1900.

These test are analog and digital TH2G.

Two TH2g (both the A and D TH2G) will run 800x600 x 6

You can also get a new digital DualHead and run 2 1920x1080 on it and 1 on the second output.

I have done this on the quatro and the x1900.

If you are just doing graphics and something like spreadsheets or pictures, you can actually get 2 TH2G boxes to display 1280x1024. Of course if you launch Catalyst or Quicktime, or anything that uses OpenGL rendering, it will immediately crash the video.