Originally Posted by AaronP
Thank you, desperadus, that's very helpful. It appears that the driver is unable to read the EDID if your "sidekick" monitor, so it's using the refresh ranges in your "Monitor1" section. You'll need to start X with "startx -- -logverbose 6" as described in the sticky post to tell for sure, but I would suspect that either there is no built-in 1280x768 mode, or it's getting rejected because it has a hsync frequency of around 47 kHz and the valid range you specified is 61 - 73.2. Also, the +1920 is not necessary in the MetaModes line unless you're using TwinView. You should be able to just remove the "metamodes" option from the "Screen1" section, because the Modes line in the Display subsection should take care of selecting the modes for that screen.
As soon as I had implemented the changes to the monitor's frequency (first putting it at 47.0-73.0 and then to 59.0-73.0 [last value from my Windows setup]) you suggested, the resolution on my 2nd display dropped down to 480x640.
No matter what other settings I tried, it was stuck there until I undid the frequency change.
Sorry, but this only makes things worse for me, not better.