Booting with only one screen powered on crashes X

What worked in feisty: fglrx driver + aticonfig --initial --dual-head ... = 2 attached monitors with each his own desktop.
I have an ATI X1600 AGP 256 Mb card, with two screens attached (one AOC 7nlr and one Compaq V50, both CRT)

Now, I've recently did a fresh install of Gusty. Here's how it went:

-Default vesa drivers wouldn't let me set refresh rate higher than 60. Restricted drivers it was, then.
-After rebooting, my X would not start.
- After a long search, I found out that it did start when I just switched on my secondary monitor. I can perfectly switch it off during GDM, and work further happily on my primary screen.

So in short:

Screen 1 turned on, Screen 2 turned on: X starts, screen 2 in mirror mode, I can switch it off perfectly and work further, unless I need to go to the login screen again.

Screen 1 turned on, Screen 2 turned off: X fails.

Screen 1 turned on, Screen 2 detached: X starts and works perfectly.

Here's my xorg.conf, which is a very simple one.

# xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by failsafeDexconf, using
# values from the debconf database and some overrides to use vesa mode.
#
# You should use dexconf or another such tool for creating a "real" xorg.conf
# For example:
# sudo dpkg-reconfigure -phigh xserver-xorg
Section "Files"
EndSection

Section "Module"
 Load "GLcore"
 Load "glx"
 Load "dbe"
 Load "v4l"
EndSection

Section "InputDevice"
 Identifier "Generic Keyboard"
 Driver "kbd"
 Option "CoreKeyboard"
 Option "XkbRules" "xorg"
 Option "XkbModel" "pc105"
 Option "XkbLayout" "be"
EndSection

Section "InputDevice"
 Identifier "Configured Mouse"
 Driver "mouse"
 Option "CorePointer"
 Option "Device" "/dev/input/mice"
 Option "Protocol" "ImPS/2"
 Option "ZAxisMapping" "4 5"
 Option "Emulate3Buttons" "true"
EndSection

Section "Device"
 Identifier "Failsafe Device"
 Boardname "vesa"
 Busid "PCI:1:0:0"
 Driver "fglrx"
 Screen 0
EndSection

Section "Monitor"
 Identifier "Failsafe Monitor"
 Vendorname "AOC"
 Modelname "AOC SPECTRUM 7Nlr"
 Horizsync 30.0-82.0
 Vertrefresh 50.0-110.0
  modeline "640x480@60" 25.2 640 656 752 800 480 490 492 525 -vsync -hsync
  modeline "640x480@72" 31.5 640 664 704 832 480 489 491 520 -vsync -hsync
  modeline "640x480@75" 31.5 640 656 720 840 480 481 484 500 -vsync -hsync
  modeline "640x480@85" 36.0 640 696 752 832 480 481 484 509 -vsync -hsync
  modeline "800x600@56" 36.0 800 824 896 1024 600 601 603 625 +hsync +vsync
  modeline "800x600@72" 50.0 800 856 976 1040 600 637 643 666 +hsync +vsync
  modeline "800x600@75" 49.5 800 816 896 1056 600 601 604 625 +hsync +vsync
  modeline "800x600@85" 56.3 800 832 896 1048 600 601 604 631 +hsync +vsync
  modeline "800x600@60" 40.0 800 840 968 1056 600 601 605 628 +hsync +vsync
  modeline "832x624@75" 57.284 832 864 928 1152 624 625 628 667 -vsync -hsync
  modeline "1024x768@85" 94.5 1024 1072 1168 1376 768 769 772 808 +hsync +vsync
  modeline "1024x768@75" 78.8 1024 1040 1136 1312 768 769 772 800 +hsync +vsync
  modeline "1024x768@70" 75.0 1024 1048 1184 1328 768 771 777 806 -vsync -hsync
  modeline "1024x768@60" 65.0 1024 1048 1184 1344 768 771 777 806 -vsync -hsync
  modeline "1024x768@43" 44.9 1024 1032 1208 1264 768 768 776 817 +hsync interlace +vsync
  modeline "1152x864@75" 108.0 1152 1216 1344 1600 864 865 868 900 +hsync +vsync
 Gamma 1.0
EndSection

Section "Screen"
 Identifier "Default Screen"
 Device "Failsafe Device"
 Monitor "Failsafe Monitor"
 Defaultdepth 24
 SubSection "Display"
 Depth 24
 Modes "1152x864@75" "1024x768@75" "800x600@75"
 EndSubSection
EndSection

Section "ServerLayout"
 Identifier "Default Layout"
  screen 0 "Default Screen" 0 0
 Inputdevice "Generic Keyboard"
 Inputdevice "Configured Mouse"
EndSection

Section "ServerFlags"
EndSection