dualhead broken with 6.12.2

[di dit]

While upgrading from version 6.9.0 to 6.12.2, dualhead stopped
working. I didn't succeed to have
the two identical monitors displaying different content. Clone mode works fine.

Trying to setup dualhead with "xrandr --output DVI-1 --left-of DVI-0"
or similar commands turn one
monitor black (standby mode?) while the other output is as expected.

I already submitted a bug to debian BTS where the relevant logs can be found
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=523589

Maybe it is related to the following bug, but I'm not sure:
http://bugs.freedesktop.org/show_bug.cgi?id=21050

lspci output:
01:00.0 VGA compatible controller: ATI Technologies Inc RV610 [Radeon
HD 2400 XT]
lspci -n output:
01:00.0 0300: 1002:94c1

xrandr --prop output:
Screen 0: minimum 320 x 200, current 1280 x 1024, maximum 2600 x 1024
DVI-1 connected 1280x1024+0+0 (normal left inverted right x axis y
axis) 376mm x 301mm
        EDID_DATA:
                00ffffffffffff0010ac264055533130
                1012010380261e782eeea5a3544c9926
                145054a54b00714f8180010101010101
                010101010101302a009851002a403070
                1300782d1100001e000000ff00555735
                333838344a303153550a000000fc0044
                454c4c203139303846500a20000000fd
                00384c1f530e000a202020202020005f
        dvi_monitor_type: auto
        scaler: off
        coherent_mode: 1 (0x00000001)   range:  (0,1)
        load_detection: 1 (0x00000001)  range:  (0,1)
   1280x1024      60.0*+   75.0     60.0*
   1152x864       75.0
   1024x768       75.0     60.0
   800x600        75.0     60.3
   640x480        75.0     59.9
   720x400        70.1
DVI-0 connected 1280x1024+0+0 (normal left inverted right x axis y
axis) 376mm x 301mm
        EDID_DATA:
                00ffffffffffff0010ac264055413330
                1012010380261e782eeea5a3544c9926
                145054a54b00714f8180010101010101
                010101010101302a009851002a403070
                1300782d1100001e000000ff00555735
                333838344a303341550a000000fc0044
                454c4c203139303846500a20000000fd
                00384c1f530e000a202020202020007f
        dvi_monitor_type: auto
        scaler: off
        coherent_mode: 1 (0x00000001)   range:  (0,1)
        load_detection: 1 (0x00000001)  range:  (0,1)
   1280x1024      60.0*+   75.0     60.0*
   1152x864       75.0
   1024x768       75.0     60.0
   800x600        75.0     60.3
   640x480        75.0     59.9
   720x400        70.1

Since dualhead has worked for me for several months before, I don't
think it is a hardware issue.

Any help welcome.