Xrandr does not support screen size across two screens

[Tobias Kaminsky]

On Thursday 27 November 2008 21:56:06 Daniil V. Kolpakov wrote:
> В сообщении от Четверг 27 ноября 2008 Tobias Kaminsky написал(a):
> > Maybe someone could tell me how to adjust the maximum.
> > Adding new modeline does not work.
>
> You propably need adjust Virtual size, like this:
>
> Section "Screen"
>     Identifier "screen1"
>     Device "device1"
>     Monitor "monitor1"
>     SubSection "Display"
>         Virtual 2560 1024
>     EndSubSection
> EndSection
>
> This should go to Xephyr's xorg.conf, I guess.

I added this to my two screen sections, it resulted in a "2x2560" screens 
where I could scroll with my mouse on both screens.
But just for testing it is okay.
The problem exists despite of that. 
1) Starting Xephyr :2
2) DISPLAY=:2 xrandr results in a maximum screen size of 1600x1200 again.

This must be wrong, or?

Thanks
Tobi