I have done plenty (too many) shows with dual TH2G. They have all been on 2008 machines with 8800s. Just because I have done this does not mean I suggest this. In fact, I won't do it anymore.

In the 2009 or 2010 machines, I have not been successful in using 2 TH2G boxes, especially mixing 2 different types.

The best recommended configuration for a TH2G is to place it on the second Output and have a monitor or 4th out on the First Output.

I will also note that 1280x1024 is 5:4 ratio not 4:3. If your output devices are 5:4 then using the 1280x1024 is reasonable. Otherwise, stick with 1024x768 unless you absolutely need the resolution. Then only use a 5:4 if your projectors are 5:4 otherwise, you'll need to use the Matrox Desktop to enable 1280x960 which is 4:3.