I want to make dual head setup without xrandr but with Xinerama.
I put "Screen 1" line into xorg.conf, but card still show identical output on DVI-2 and DVI-3
It is important to use xinerama for me (to glue three monitors), that's why i decide not to use ranrd (randr is incompatible with xinerama as i read somewhere)

Here is my videocard (HD 4850 X2):

lspci | grep R700

03:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI R700 [Radeon HD 4850]
04:00.0 Display controller: Advanced Micro Devices [AMD] nee ATI R700 [Radeon HD 4850]

Here is how monitors are connected:

grep "DVI" /var/log/Xorg.0.log

[  1210.002] (II) RADEON(0): Output DVI-0 using monitor section Monitor0
[  1210.048] (II) RADEON(0): Output DVI-1 has no monitor section
[  1210.079] (II) RADEON(0): EDID for output DVI-0
[  1210.080] (II) RADEON(0): Printing probed modes for output DVI-0
[  1210.128] (II) RADEON(0): EDID for output DVI-1
[  1210.128] (II) RADEON(0): Output DVI-0 connected
[  1210.128] (II) RADEON(0): Output DVI-1 disconnected
[  1210.128] (II) RADEON(0): Output DVI-0 using initial mode 1920x1200
[  1210.160] (II) RADEON(1): Output DVI-2 using monitor section Monitor2
[  1210.215] (II) RADEON(1): Output DVI-3 has no monitor section
[  1210.246] (II) RADEON(1): EDID for output DVI-2
[  1210.247] (II) RADEON(1): Printing probed modes for output DVI-2
[  1210.299] (II) RADEON(1): EDID for output DVI-3
[  1210.300] (II) RADEON(1): Printing probed modes for output DVI-3
[  1210.300] (II) RADEON(1): Output DVI-2 connected
[  1210.300] (II) RADEON(1): Output DVI-3 connected
[  1210.300] (II) RADEON(1): Output DVI-2 using initial mode 1920x1200
[  1210.300] (II) RADEON(1): Output DVI-3 using initial mode 1920x1200

Here is my /etc/X11/xorg.conf

Section "ServerFlags"  
    Option        "RandR"   "0"  
    Option        "Xinerama" "1"  
EndSection  

Section "ServerLayout"  
    Identifier     "Three Head Layout"  

    Screen        "MyPrecious0" 
    Screen        "MyPrecious2" RightOf "MyPrecious0"
    Screen        "MyPrecious3" LeftOf "MyPrecious0"
EndSection

Section "Screen"
    Identifier "MyPrecious0"
    Monitor    "Monitor0"
    Device     "Device300"
EndSection

Section "Screen"
    Identifier "MyPrecious2"
    Monitor    "Monitor2"
    Device     "Device400"
EndSection

Section "Screen"
    Identifier "MyPrecious3"
    Monitor    "Monitor3"
    Device     "Device401"
EndSection

Section "Device"
    Identifier  "Device300"
    BusID      "PCI:3:0:0"

    Screen 0
    Driver     "radeon"
EndSection

Section "Device"
    Identifier  "Device400"
    BusID       "PCI:4:0:0"

    Screen 0
    Driver      "radeon"
EndSection

Section "Device"
    Identifier  "Device401"
    BusID       "PCI:4:0:0"

    Screen 1
    Driver      "radeon"
EndSection

Section "Monitor"
    Identifier   "Monitor0"
EndSection

Section "Monitor"
    Identifier   "Monitor2"
EndSection

Section "Monitor"
    Identifier   "Monitor3"
EndSection

I tried to switch to vesa driver (didn't work for me)

I tried to add options like Option "ZaphodHeads" "DVI-2" and Option "ZaphodHeads" "DVI-3" into sections "Device 400" and "Device 401" (this didn't help because "ZaphodHeads" option is for ranrd, and randr is disabled by decision)

I tried to merge sections "Device 400" and "Device 401" into one section and add Option "ZaphodHeads" "DVI-2,DVI-3" (see comment about randr above) single section setup helps to change log line RADEON(1): Output DVI-3 has no monitor section into RADEON(1): Output DVI-3 using monitor section Monitor3

but nothing was enough to switch from screen cloning to separate screens.

This problem (lack of documentation on radeon driver) is similar to these: Radeon display driver clones monitors while using Xinerama (moderators decision to close that problem was wrong) Ubuntu 12.10 multi-monitor setup isn't working

The problem is solvable, because this hardware worked as three headed
for me earlier with gentoo/xorg-server-1.3

Xorg -configure

creates setup for the first monitor on the first GPU

Please don't advise to use fglrx/aticonfig/amdcccle (this goes against my religion beliefs)