How can I use two video cards to power three screens?

I have two Radeon 6450 graphics cards in my computer, with three screens. Only the screens plugged into the first graphics card are recognized and configurable in the displays setting panel. How can I use the displays plugged into my second graphics card?

Both cards are present in lspci:

01:00.0 VGA compatible controller: ATI Technologies Inc NI Caicos [AMD RADEON HD 6450]
01:00.1 Audio device: ATI Technologies Inc NI Caicos HDMI Audio [AMD RADEON HD 6450]
02:00.0 VGA compatible controller: ATI Technologies Inc NI Caicos [AMD RADEON HD 6450]
02:00.1 Audio device: ATI Technologies Inc NI Caicos HDMI Audio [AMD RADEON HD 6450]

I'm using the open source driver instead of the proprietary driver because the proprietary driver won't output 1440x2560.

Solution 1:

It's possible.

Xorg - The Graphical user environment supports multiple separate and or different graphic cards displaying output)

The feature is incompatible with Unity though. You will need to run a different desktop environment

Its also quite bothersome to configure, since you have to configure it manually in /etc/Xorg.conf file.this thread Has a fairly detailed description of how to do it. Prepare for some work!

Its old but Xorg.conf hasn't changed for years.

Solution 2:

Someone made a similar thing with nvidia graphic boards: https://cornerstone.multitouch.fi/cornerstone-documentation/multiple-graphics-cards maybe some of the steps are of some help. Instead of nvidia-settings you may try aticonfig.