As Nev said: Catalyst only supports ONE graphics card
You should be able to use a DH2G on the 2nd Dual-Link DVI output and get 3840x1200 (2 x 1920x1200 outputs) or lower - and still use your primary output for another 1920x1200 output.
2009 macs only seem to support 1xDual-Link and 1xSingle-Link output natively with the current available 'supported' graphics cards. You should ensure that you use the 'Dual-link' DVI output to connect to the DH2G or TH2G, rather than the 'single link' Mini Display-port output.
Alternatively, to achieve your desired resolutions - test to see if your screens can accept a 1024x768 image, and then stretch to fit width. - If they can - you can use a TH2G at 3072x768 and run the content 'anamorphic' in catalyst, in 3 mix windows, and allow the screens to stretch the image back to a 16:9 ratio (1360x768).
Contact me further for explanation if this is an option for you.
Recently I have run 2 x TH2G on a 2009 Mac Pro with an either ATI HD4870 or Nvidia 880GT (10.4.7). With one TH2G on each output, I achieved:
Single Link / Dual Link
1x 2400x800 + 1x 2400x800 (6x 800x600)
1x 2560x1024* + 1x3840x1024 (5x1280x1024)
1x 2048x768* + 1x 3072x768 (5x1024x768)
(*Triple-Head2Go in Dual-Head mode)
You can use 2 separate Macs to send to separate screens. It is possible to sync with MIDI if you are running Midi Timecode anyway. Otherwise - use the sync ID function to syncronise between layers on each server.
Do contact me if you require further assistance
Regards
Simon