Problem setting up two Screens on two 30-inch displays

Hi,

I'm trying to set up psychtoolbox to run two 30-inch displays with ubuntu.

I'm able to open a screen and draw to each of them separately without issue. But when I try to open one screen in both of them at the same time I get the following error:

PTB-ERROR:[glXCreateContext() failed] OpenGL context creation failed!

PTB-ERROR[Low-Level setup of window failed]:The specified display may not support double buffering and/or stereo output. There could be insufficient video memory

I am using a Nvidia GTX 280 with 1Gb of memory and the two displays are connected and function properly through DVI. I'm using Ubuntu 12.04 LTE with the official Nvidia drivers (295.40), and the displays are configured as 'separate X screens'.

In order to address that issue, I tried to plug one of the two monitors through a Nvidia GTX 285. But the same PTB-ERROR appeared.

Do you have any thoughts on how to solve that issue?

(By configuring the two 'separate X screens' in ubuntu one of them is gray, but I can use it without problem from psychtoolbox)

Thanks!
Adria