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I currently have a Radeon HD 5750 graphics card and an M3N78-PRO motherboard. I use both outputs of my graphics card for a 2 screen display. I use xrandr at boot to set up my two screens.

However, I was wondering if (1) would it be possible to attach a third monitor to the HDMI output on my motherboard and (2) if it is, how I can use xrandr to set up the third monitor correctly.

Hardware-wise, I don't know if this is possible (I sure hope so). If it is possible, will there be any confusion at the OS level as to where to send the display (motherboard vs. graphics card)?

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up vote 2 down vote accepted

Current version of RandR protocol does not allow to use multiple graphic devices. It is expected to be implemented in RandR 2.0, but there is no estimate time of release (so do not expect it in the next two years). You might want to see (for example) here: http://en.wikipedia.org/wiki/RandR

However, it might be possible to configure your monitors without RandR, using the deprecated Xinerama extension. Unfortunately I know close to nothing about Xinerama, so I cannot help you any further.

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Thanks, I didn't know that. +1 and accepted -- I'll have to find another way to do this. – n0pe Mar 16 '12 at 15:31
Just to note that the currently accepted answer is actually incorrect. RandR 1.4 supports multiple GPUs. – Martin Eve Apr 9 '14 at 11:06
Yes, obviously at the time of writing this answer the most recent RandR version was 1.3. I haven't had hands-on experience with 1.4 yet. – Paweł Rumian Apr 9 '14 at 14:51

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