Xrandr does not support screen size across two screens

[Tobias Kaminsky]

Hey,

I am starting X across two screens using two graphic adapters.
When I load Xephyr into it, I cannot adjust the size with xrandr to the 
maximum of 2560x1024 (2 tfts of 1280x1024).
Xrandr's output says that the maximum is 1600x1200 (which could be the maximum 
of 1! tft, I guess).

Xrandr should at least allow to set Xephyr to the real size of X, or?

Maybe someone could tell me how to adjust the maximum.
Adding new modeline does not work.

Thanks
Tobi