I'm using Debian Jessie with two graphic cards and 3 Monitors. One on the onboard card and two on an internal card. I can select and configure the two monitors on the internal card but can't get to the monitor on the onboard graphics card. Looking at the Xorg.log with my limited abilities to read it, I think both cards are activated and accessible. My Xorg.log (I couldn't paste it here): https://gist.github.com/anonymous/b67736d974f1d5f23a19
Yet I can't configure it with the provided gui tool. But also xrandr does not show the monitor. I read that I could be able to set it up with this program.
Screen 0: minimum 320 x 200, current 3200 x 1080, maximum 8192 x 8192
VGA1 connected 1280x1024+1920+0 (normal left inverted right x axis y axis) 338mm x 270mm
1280x1024 60.02*+ 75.02
1024x768 75.08 70.07 60.00
832x624 74.55
800x600 72.19 75.00 60.32 56.25
640x480 75.00 72.81 66.67 60.00
720x400 70.08
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 160mm x 90mm
1920x1080 60.00 + 50.00 59.94*
1920x1080i 60.00 50.00 59.94
1600x1200 60.00
1680x1050 59.88
1280x1024 60.02
1440x900 59.90
1280x960 60.00
1280x800 59.91
1280x720 60.00 50.00 59.94
1024x768 60.00
800x600 60.32 56.25
720x576 50.00
720x480 60.00 59.94
640x480 60.00 59.94