I noticed the same problem with Matrox G550 and driver xserver-xorg-video-mga 1:1.4.7.dfsg.1-3, only the first monitor works while Xorg.0.log does not report any problems about the second.
Trying with the driver version from the experimental (1:1.9.99.dfsg.1-1) with the same config gives some kind of clone mode with weird resolutions for two monitors with several graphical glitches. I tried to change the config with grandr but it did not give any other choice than the clone mode, but this might be fault of grandr and not the driver. The man-page of xrandr does not look very inviting so I did not yet try changing the settings with it, and instead downgraded the driver and some other xorg packages to the versions in testing to get a working set up. Are these newer drivers likely to get fixed anytime soon or is the "medium term" solution to stay with 1.4.6 driver? I am personally not very motivated to try to fix or hunt for bugs in the experimental mga driver (good reason to get rid of the POS matrox if anything), and maybe nobody else is either for such old cards... -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]