Mmmmh.
But how come I can have 1920x1080 and 3840x1024 working fine when no external devices are attached? My monitors are fine with all of that, which suggests the x1900 is fine with that also. I have made some progress in that I have outputs 1+3 on the TH2G working (with or without 1920x1080 on the other DVI out), but nothing apart from the local monitor wants to display the centre 1280x1024.