I have an IBM Thinkpad T42 with a 1400x1050 internal screen connected to an ATI Mobility Radeon 9600 M10 GPU (RV350), which I previously used in a dualhead configuration with an external Samsung SyncMaster 920T TFT screen. I enabled MergedFB with a "1400x1050-1280x1024" MetaMode and this setup worked well.

Now I replaced this external display with another Samsung flatscreen: a SyncMaster 940BW, which is a wide-screen display with 1440x900 pixels. I added this new resolution to the Screen section and added an appropriate MetaMode for MergedFB: "1400x1050-1440x900".

The X server (X.org 7.2 on an openSUSE 10.2 installation, xorg-x11-7.2-146.7) initially did not accept this mode with the following message:

(WW) RADEON(0): Mode "1440x900" is not a supported mode for CRT2
(WW) RADEON(0):         (Skipping metamode "1400x1050-1440x900")

The second head was then driven with a lower resolution (1024x768). I was able to resolve this problem by manually adding a matching mode line to the Modes section:

  Modeline  "1440x900" 106.5 1440 1520 1672 1904 900 903 909 934

This finally enabled the correct resolution on the second head. You can see the detailed history of this issue in the openSUSE xfree86 Mailing list (see the URL field added to this report) - not sure if it's worth a separate bug report (as I think the server should have detected the appropriate resolution via DDC itself).

However, I now face one remaining problem: as the vertical resolution on the external display is lower than the one on the internal panel (900 vs. 1050 pixel), the screen on the second head now scrolls up and down when I move the mouse over the upper or lower screen border. Which is strange, as this did not happen when using the previous monitor (which had 1024 pixel vertically, which is still less than the 1050 of the internal one).

According to the radeon man page this should be fixed be enabling the MergedNonRectangular option, which I already had included in the configuration before:

  Option       "MergedNonRectangular" "true"

Unfortunately the screen still scrolls, which is slightly annoying and I consider this a bug, given that it worked with the other display. The xorg.conf and X log file is attached. Let me know if you need any additional info! Thanks.