I have a ATI Radeon 2400 XT, and a Nvidia GTX 580 in my debian computer. The 580 has 3 ports, but only 2 of them could be used at the same time. I bought the refurbished Radeon so that I could use another screen, but it was being ignored.
I reconfigured my bios so that the Radeon was the primary display, and the ttys now use that display.
After more fiddling, I managed to get my computer to show the cursor in the third screen when I moved my mouse into it, but in Gnome windows do not move with it. I have also added the ppa xorg-edgers
I stopped gdm, and tried with xinit, xterm and openbox. I had the same problem.
I looked at:
I could not find anything for debian except for how to setup each individual graphics card.
I am using the free xserver-xorg-video-radeon driver and the proprietary nvidia-driver
xrandr does not detect the Radeon GPU, but lspci and X do.
The gnome cursor passes between them
After looking at https://askubuntu.com/questions/593938, it almost works. Interactions with windows still works, and so does the mouse. However, the graphics do not transfer, and I am left with a glitched screen.